CSS Scroll Snap Module Level 1

W3C Candidate Recommendation Snapshot,

This version:
https://www.w3.org/TR/2021/CR-css-scroll-snap-1-20210311/
Latest published version:
https://www.w3.org/TR/css-scroll-snap-1/
Editor's Draft:
https://drafts.csswg.org/css-scroll-snap-1/
Previous Versions:
Implementation Report:
https://wpt.fyi/results/css/css-scroll-snap
Test Suite:
http://test.csswg.org/suites/css-scroll-snap-1_dev/nightly-unstable/
Issue Tracking:
CSSWG Issues Repository
Editors:
Matt Rakow (Microsoft)
Jacob Rossi (Microsoft)
Tab Atkins-Bittner (Google)
Elika J. Etemad / fantasai (Invited Expert)
Suggest an Edit for this Spec:
GitHub Editor

Abstract

This module contains features to control panning and scrolling behavior with “snap positions”.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document was published by the CSS Working Group as a Candidate Recommendation Snapshot. Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. A Candidate Recommendation Snapshot has received wide review and is intended to gather implementation experience. This document is intended to become a W3C Recommendation; it will remain a Candidate Recommendation at least until to gather additional feedback.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-scroll-snap” in the title, like this: “[css-scroll-snap] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 15 September 2020 W3C Process Document.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

A test suite and an implementation report will be produced during the CR period.

The following features are at-risk, and may be dropped during the CR period:

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

1. Introduction

This section is not normative.

Popular UX paradigms for scrollable content frequently employ paging through content, or sectioning into logical divisions. This is especially true for touch interactions where it is quicker and easier for users to quickly pan through a flatly-arranged breadth of content rather than delving into a hierarchical structure through tap navigation. For example, it is easier for a user to view many photos in a photo album by panning through a photo slideshow view rather than tapping on individual photos in an album.

However, given the imprecise nature of scrolling inputs like touch panning and mousewheel scrolling, it is difficult for web developers to guarantee a well-controlled scrolling experience, in particular creating the effect of paging through content. For instance, it is easy for a user to land at an awkward scroll position which leaves an item partially on-screen when panning.

To this end, this module introduces scroll snap positions which enforce the scroll positions that a scroll container’s scrollport may end at after a scrolling operation has completed.

Also, to offer better control over paging and scroll positioning even when snapping is off, this module defines the scroll-padding property for use on all scroll containers, to adjust the scroll container’s optimal viewing region for the purpose of paging and scroll-into-view operations. Similarly the scroll-margin property can be used on any box to adjust its visual area for the purpose of scroll-into-view operations.

1.1. Module interactions

This module extends the scrolling user interface features defined in [CSS2] section 11.1.

None of the properties in this module apply to the ::first-line and ::first-letter pseudo-elements.

1.2. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

2. Motivating Examples

In this example, a series of images arranged in a scroll container are used to build a photo gallery. In this example the scroll container is larger than the photos contained within (such that multiple images may be seen simultaneously), and the image sizes vary. Using mandatory element-based snap positions, scrolling will always complete with an image centered in the scroll container’s scrollport.
img {
    /* Specifies that the center of each photo
       should align with the center of the scroll
       container in the X axis when snapping */
    scroll-snap-align: none center;
}
.photoGallery {
    width: 500px;
    overflow-x: auto;
    overflow-y: hidden;
    white-space: nowrap;
    /* Requires that the scroll position always be
       at a snap position when the scrolling
       operation completes. */
    scroll-snap-type: x mandatory;
}
<div class="photoGallery">
    <img src="img1.jpg">
    <img src="img2.jpg">
    <img src="img3.jpg">
    <img src="img4.jpg">
    <img src="img5.jpg">
</div>
The layout of the scroll container’s contents in the example. The snapport is represented by the red rectangle, and the snap area is represented by the yellow rectangle. Since the scroll-snap-align is “center” in the inline (horizontal) axis, a snap position is established at each scroll position which aligns the X-center of the snapport (represented by a red dotted line) with the X-center of a snap area (represented by a yellow dotted line).
This example builds a paginated document that aligns each page near to (but not exactly on) the edge of the scroll container. This allows the previous page to “peek” in from above in order to make the user aware that they are not yet at the top of the document. Using proximity snap positions instead of mandatory snap positions allows the user to stop halfway through a page (rather than forcing them to snap one page at a time). However, if a scrolling operation would finish near a snap position, then the scroll will be adjusted to align the page as specified.
.page {
    /* Defines the top of each page as the
       edge that should be used for snapping */
    scroll-snap-align: start none;
}
.docScroller {
    width: 500px;
    overflow-x: hidden;
    overflow-y: auto;
    /* Specifies that each element’s snap area should
       align with a 100px offset from the top edge. */
    scroll-padding: 100px 0 0;
    /* Encourages scrolling to end at a snap position when the
        operation completes, if it is near a snap position */
    scroll-snap-type: y proximity;
}
<div class="docScroller">
    <div class="page">Page 1</div>
    <div class="page">Page 2</div>
    <div class="page">Page 3</div>
    <div class="page">Page 4</div>
</div>
The layout of the scroll container’s contents in the example. The snapport is represented by the red rectangle (inset from the top by 100px due to the scroll-padding), and the snap area is represented by the yellow rectangle. Since the scroll-snap-align is “start” in the Y axis, a snap position is established at each scroll position which aligns the Y-start of the snapport (represented by a red dotted line) with the Y-start of a snap area (represented by a yellow dotted line).

3. Scroll Snap Model

This module defines controls for scroll snap positions, which are scroll positions that produce particular alignments of content within a scroll container. Using the scroll-snap-type property on the relevant scroll container, the author can request a particular bias for the scrollport to land on a snap position after scrolling operations (including programmatic scrolls such as the scrollTo() method).

Snap positions are specified as a particular alignment (scroll-snap-align) of an element’s scroll snap area (its border bounding box, as modified by scroll-margin) within the scroll container’s snapport (its scrollport, as reduced by scroll-padding). This is conceptually equivalent to specifying the alignment of an alignment subject within an alignment container. A scroll position that satisfies the specified alignment is a snap position.

The act of adjusting the scroll position of a scroll container’s scrollport such that it is aligned to a snap position is called snapping, and a scroll container is said to be snapped to a snap position if its scrollport’s scroll position is that snap position and there is no active scrolling operation. The CSS Scroll Snap Module intentionally does not specify nor mandate any precise animations or physics used to enforce snap positions; this is left up to the user agent.

Snap positions only affect the nearest ancestor scroll container on the element’s containing block chain.

4. Capturing Scroll Snap Areas: Properties on the scroll container

4.1. Scroll Snapping Rules: the scroll-snap-type property

Name: scroll-snap-type
Value: none | [ x | y | block | inline | both ] [ mandatory | proximity ]?
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: specified keyword(s)
Canonical order: per grammar
Animation type: discrete

The scroll-snap-type property specifies whether a scroll container is a scroll snap container, how strictly it snaps, and which axes are considered. If no strictness value is specified, proximity is assumed.

In this example, snapping to headings is enabled in the block axis (the y axis for horizontal writing, x axis for vertical writing):
html {
  scroll-snap-type: block;   /* applied to main document scroller */
}
h1, h2, h3, h4, h5, h6 {
  scroll-snap-align: start;  /* snap to the start (top) of the viewport */
}

UAs must apply the scroll-snap-type value set on the root element to the document viewport. Note that, unlike overflow, scroll-snap-type values are not propagated from HTML body.

4.1.1. Scroll Snap Axis: the x, y, block, inline, and both values

The axis values specify what axis(es) are affected by snap positions, and whether snap positions are evaluated independently per axis, or together as a 2D point. Values are defined as follows:

x
The scroll container snaps to snap positions in its horizontal axis only.
y
The scroll container snaps to snap positions in its vertical axis only.
block
The scroll container snaps to snap positions in its block axis only.
inline
The scroll container snaps to snap positions in its inline axis only.
both
The scroll container snaps to snap positions in both of its axes independently (potentially snapping to different elements in each axis).

4.1.2. Scroll Snap Strictness: the none, proximity, and mandatory values

The strictness values (none, proximity, mandatory) specify how strictly snap positions are enforced on the scroll container (by forcing an adjustment to the scroll position). Values are defined as follows:

none
If specified on a scroll container, the scroll container must not snap.
mandatory
If specified on a scroll container, the scroll container is required to be snapped to a snap position when there are no active scrolling operations. If a valid snap position exists then the scroll container must snap at the termination of a scroll (if none exist then no snapping occurs).
proximity
If specified on a scroll container, the scroll container may snap to a snap position at the termination of a scroll, at the discretion of the UA given the parameters of the scroll.

Authors should use mandatory snap positions with consideration of varyingly-sized screens and (if applicable) varying-sized content. In particular, although access to snapped elements larger than the scrollport is handled by the UA, if authors assign mandatory snapping to non-adjacent siblings, content in between can become inaccessible in cases where it is longer than the screen.

A box captures snap positions if it is a scroll container or has a value other than none for scroll-snap-type. If a box’s nearest snap-position capturing ancestor on its containing block chain is a scroll container with a non-none value for scroll-snap-type, that is the box’s scroll snap container. Otherwise, the box has no scroll snap container, and its snap positions do not trigger snapping.

4.1.3. Re-snapping After Layout Changes

If the content or layout of the document changes (e.g. content is added, moved, deleted, resized) such that the content of a snapport changes, the UA must re-evaluate the resulting scroll position, and re-snap if required. If the scroll container was snapped before the content change and that same snap position still exists (e.g. its associated element was not deleted), the scroll container must be re-snapped to that same snap position after the content change. If multiple boxes were snapped before and their snap positions no longer coincide, then if one of them is focused or targeted, the scroll container must re-snap to that one and otherwise which one to re-snap to is UA-defined. (The UA may, for example, track which element is snapped as layout shifts align and de-align the snap positions of other elements.)

Scrolling required by a re-snap operation to a new or different box must behave and animate the same way as any other scroll-into-view operation, including honoring controls such as scroll-behavior. Scrolling behavior for re-snapping to the same box as before however, is UA-defined. The UA may, for example, when snapped to the start of a section, choose not to animation the scroll to the section’s new position as content is dynamically added earlier in the document in order to create the illusion of not scrolling.

In the following example, the log console, when initially loaded and as each message is added to the bottom, remains snapped to the bottom of the content unless the user has scrolled away from that edge:
.log {
  scroll-snap-type: proximity;
  align-content: end;
}
.log::after {
  display: block;
  content: "";
  scroll-snap-align: end;
}

The rules create a single scroll snap area represented by the ::after pseudo-element, positioned at the very bottom of a scroll snap container. If the user scrolls “near” the bottom, the container will snap to it. If more content is dynamically added to the container, it’ll remain snapped to it (because scroll containers are required to re-snap to the same scroll snap area if it still exists after any changes). However, if the user has scrolled to somewhere else in the logs, it won’t do anything at all.

4.2. Scroll Snapport: the scroll-padding property

Name: scroll-padding
Value: [ auto | <length-percentage> ]{1,4}
Initial: auto
Applies to: scroll containers
Inherited: no
Percentages: relative to the corresponding dimension of the scroll container’s scrollport
Computed value: per side, either the keyword auto or a computed <length-percentage> value
Animation type: by computed value type
Canonical order: per grammar

This property specifies (for all scroll containers, not just scroll snap containers) offsets that define the optimal viewing region of a scrollport: the region used as the target region for placing things in view of the user. This allows the author to exclude regions of the scrollport that are obscured by other content (such as fixed-positioned toolbars or sidebars) or simply to put more breathing room between a targeted element and the edges of the scrollport.

The scroll-padding property is a shorthand property that sets all of the scroll-padding-* longhands in one declaration, assigning values to the longhands representing each side exactly as the padding property does for its longhands. Values have the following meanings:

<length-percentage>

Defines an inward offset from the corresponding edge of the scrollport. When applied to the root viewport, the offset is calculated and applied relative to the layout viewport (rather than the visual viewport) the same way as the corresponding inset properties on fixed-positioned boxes; the optimal viewing region is the remaining area that intersects with the visual viewport.

auto

Indicates that the offset for the corresponding edge of the scrollport is UA-determined. This should generally default to a used length of 0px, but UAs may use heuristics to detect when a non-zero value is more appropriate.

For example, a UA could detect when a position:fixed element is being used as an opaque unscrollable “header” that obscures the content below it, and resolve the top offset to the height of that element so that a “page down” operation (such as pressing PgDn) automatically scrolls by one “visible page” of content.

These offsets reduce the region of the scrollport that is considered “viewable” for scrolling operations: they have no effect on layout, on the scroll origin or initial position, or on whether or not an element is considered actually visible, but should affect whether an element or the caret is considered scrolled into view (e.g. for targeting or focusing operations), and reduce the amount of scrolling for paging operations (such as using the PgUp and PgDn keys or triggering equivalent operations from the scrollbar) so that within the optimal viewing region of the scrollport the user sees a continuous stream of content.

For a scroll snap container this region also defines the scroll snapportthe area of the scrollport that is used as the alignment container for the scroll snap areas when calculating snap positions.

In this example, scroll-padding is used to center slideshow images within the portion of the scrollport that is not obscured by a fixed-position toolbar.
html {
    overflow-x: auto;
    overflow-y: hidden;
    scroll-snap-type: x mandatory;
    scroll-padding: 0 500px 0 0;
}
.toolbar {
    position: fixed;
    height: 100%;
    width: 500px;
    right: 0;
}
img {
    scroll-snap-align: none center;
}

UAs must apply the scroll-padding values set on the root element to the document viewport. (Note that, unlike overflow, scroll-padding values are not propagated from HTML body.)

5. Aligning Scroll Snap Areas: Properties on the elements

5.1. Scroll Snapping Area: the scroll-margin property

Name: scroll-margin
Value: <length>{1,4}
Initial: 0
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: per side, an absolute length
Canonical order: per grammar
Animation type: by computed value type

This property is a shorthand property that sets all of the scroll-margin-* longhands in one declaration, assigning values to the longhands representing each side exactly as the margin property does for its longhands.

Values represent outsets defining the scroll snap area that is used for snapping this box to the snapport. The scroll snap area is determined by taking the transformed border box, finding its rectangular bounding box (axis-aligned in the scroll container’s coordinate space), then adding the specified outsets.

Note: This ensures that the scroll snap area is always rectangular and axis-aligned to the scroll container’s coordinate space.

If a page is navigated to a fragment that defines a target element (one that would be matched by :target, or the target of scrollIntoView()), the UA should use the element’s scroll snap area, rather than just its border box, to determine which area of the scrollable overflow area to bring into view, even when snapping is off or not applied on this element.

5.2. Scroll Snapping Alignment: the scroll-snap-align property

Name: scroll-snap-align
Value: [ none | start | end | center ]{1,2}
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: two keywords
Canonical order: per grammar
Animation type: discrete

The scroll-snap-align property specifies the box’s snap position as an alignment of its snap area (as the alignment subject) within its snap container’s snapport (as the alignment container). The two values specify the snapping alignment in the block axis and inline axis, respectively, as determined by the snap container’s writing mode. If only one value is specified, the second value defaults to the same value.

Values are defined as follows:

none
This box does not define a snap position in the specified axis.
start
Start alignment of this box’s scroll snap area within the scroll container’s snapport is a snap position in the specified axis.
end
End alignment of this box’s scroll snap area within the scroll container’s snapport is a snap position in the specified axis.
center
Center alignment of this box’s scroll snap area within the scroll container’s snapport is a snap position in the specified axis.

Start and end alignments are resolved with respect to the writing mode of the snap container unless the scroll snap area is larger than the snapport, in which case they are resolved with respect to the writing mode of the box itself. (This allows items in a container to have consistent snap alignment in general, while ensuring that start always aligns the item to allow reading its contents from the beginning.)

5.2.1. Scoping Valid Snap Positions to Visible Boxes

Since the purpose of scroll snapping is to align content within the scrollport for optimal viewing, a scroll position cannot be considered a valid snap position if snapping to it would leave the contributing snap area entirely outside the snapport, even if it otherwise satisfies the required alignment of the snap area.

For example, a snap area is top-aligned to the snapport if its top edge is coincident with the snapport’s top edge; and this would be considered a valid snap position for block-axis start-aligned snapping of that snap area if at least part of the snap area is on-screen. If the entire snap area is outside the snapport, however, then the scroll container cannot be considered to be snapped because the required alignment, though satisfied, would not be relevant to the viewer.
╔════viewport════╗┈┈┈┈┈┈┈┈┌──────────────┐
║  ┌─────┐ ┌──┐  ║        │ top-snapping │
║  ├──┐  │ └──┘  ║        │   element    │
║  └──┴──┘       ║        │              │
╚════════════════╝        │              │
                          └──────────────┘
Alignment of an off-screen element is not considered snapping.
Why limit snapping to only when the element is visible? As the WebKit implementers point out, extending a snap edge infinitely across the canvas only allows for snapping gridded layouts, and produces odd behavior for the user when off-screen elements do not align with on-screen elements. (If this requirement is onerous for implementers however, we can default to a gridded behavior and introduce a switch to get smarter behavior later.)

Note: Although scroll-snap-type: both evaluates snap positions independently in each axis, choosing of a snap position in one axis can be influenced by snap positions in the other axis. For example, snapping in one axis may push off-screen the snap area that the other axis would otherwise align to, making its snap position invalid and therefore unchooseable.

5.2.2. Snapping Boxes that Overflow the Scrollport

If the snap area is larger than the snapport in a particular axis, then any scroll position in which the snap area covers the snapport, and the distance between the geometrically previous and subsequent snap positions in that axis is larger than size of the snapport in that axis, is a valid snap position in that axis. The UA may use the specified alignment as a more precise target for certain scroll operations (e.g. explicit paging).

For example, take the first example in § 2 Motivating Examples, which had a photo as the area. The author wants mandatory snapping from item to item, but if the item happens to be larger than your viewport, you want to be able to scroll around the whole thing once you’re over it.

Since the snap area is larger than the snapport, while the area fully fills the viewport, the container can be scrolled arbitrarily and will not try to snap back to its aligned position. However, if the container is scrolled such that the area no longer fully fills the viewport in an axis, the area resists outward scrolling until it is scrolled sufficiently to trigger snapping to a different snap position.

For another example, mandatory top-snapping on nested section elements can produce large snapping areas (from large top-level sections) potentially filled with smaller snapping areas (from the subsections). When the subsections are small enough, they snap normally; when they’re longer, the viewer can scroll arbitrarily within them, or within a large segment of the top-level section that has no subsections to snap to.
┌─ top-level section ─┐ ━┓
│                     │ 1┃
│                     │  ┃
│                     │ ━┩
│                     │  ┆
│                     │  ┆
│┌─── sub-section ───┐│  ╯ ━┓
│└───────────────────┘│    2┃
│┌─── sub-section ───┐│ ━┓  ┃
││                   ││ 3┃ ━┛
│└───────────────────┘│  ┃
│┌─── sub-section ───┐│ ━┛ ━┓
│└───────────────────┘│    4┃
│┌─── sub-section ───┐│ ━┓  ┃
││                   ││ 5┃ ━┛
││                   ││  ┃
││                   ││ ━┩
││                   ││  ┆
││                   ││  ┆
││                   ││  ┆
│└───────────────────┘│  ┆
└─────────────────────┘  ╯
In the figure above, the five numbered viewports represent the five snap positions associated with the top-level section and its four subsections. Because the first and last snap positions are part of ranges taller than the viewport, the viewer is allowed to scroll freely between the top and bottom of each range.

Note: If the author had instead set mandatory snap positions on the headings of each section (rather than the sections themselves), the contents of the first and fifth sections would be partially inaccessible to the user, as the heading snap area does not extend to cover the whole section. This is why it’s a bad idea to use mandatory snap positions on elements that might be widely spaced apart.

5.2.3. Unreachable Snap Positions

If a snap position is unreachable as specified, such that aligning to it would require scrolling the scroll container’s viewport past the edge of its scrollable overflow area, the used snap position for this snap area is the position resulting from scrolling as much as possible in each relevant axis toward the desired snap position.

5.3. Scroll Snap Limits: the scroll-snap-stop property

Name: scroll-snap-stop
Value: normal | always
Initial: normal
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: specified keyword
Canonical order: per grammar
Animation type: discrete

When scrolling with an intended direction, the scroll container can “pass over” several possible snap positions (that would be valid to snap to, if the scrolling operation used the same direction but a lesser distance) before reaching the natural endpoint of the scroll operation and selecting its final scroll position. The scroll-snap-stop property allows such a possible snap position to “trap” the scrolling operation, forcing the scroll container to stop before the scrolling operation would naturally end.

Values are defined as follows:

normal
The scroll container may pass over a snap position defined by this element during the execution of a scrolling operation.
always
The scroll container must not pass over a snap position defined by this element during the execution of a scrolling operation; it must instead snap to the first of this element’s snap positions.

This property has no effect on scrolling operations with only an intended end position, as they do not conceptually “pass over” any snap positions.

6. Snapping Mechanics

The precise model algorithm to select a snap position to snap to is intentionally left mostly undefined, so that user agents can take into account sophisticated models of user intention and interaction and adjust how they respond over time, to best serve the user.

This section defines some useful concepts to aid in discussing scroll-snapping mechanics, and provides some guidelines for what an effective scroll-snapping strategy might look like. User agents are encouraged to adapt this guidance and apply their own best judgement when defining their own snapping behavior. It also provides a small number of behavior requirements, to ensure a minimum reasonable behavior that authors can depend on when designing their interfaces with scroll-snapping in mind.

6.1. Types of Scrolling Methods

When a page is scrolled, the action is performed with an intended end position and/or an intended direction. Each combination of these two things defines a distinct category of scrolling, which can be treated slightly differently:

intended end position
Common examples of scrolls with only an intended end position include:
  • a panning gesture, released without momentum

  • manipulating the scrollbar “thumb” explicitly

  • programmatically scrolling via APIs such as scrollTo()

  • tabbing through the document’s focusable elements

  • navigating to an anchor within the page

  • homing operations such as the Home/End keys

intended direction and end position
Common examples of scrolls with both an intended direction and end position include:
  • a “fling” gesture, interpreted with momentum

  • programmatically scrolling via APIs such as scrollBy()

  • paging operations such as the PgUp/PgDn keys (or equivalent operations on the scrollbar)

The intended end point of the scroll prior to intervention from features such as snap points is its natural end-point.

intended direction
Common examples of scrolls with only an intended direction include:
  • pressing an arrow key on the keyboard (or equivalent operations on the scrollbar)

  • a swiping gesture interpreted as a fixed (rather than inertial) scroll

Additionally, because page layouts usually align things vertically and/or horizontally, UAs sometimes axis-lock a scroll when its direction is sufficiently vertical or horizontal. An axis-locked scroll is bound to only scroll along that axis. This prevents less-precise input mechanisms from drifting in the non-primary axis.

Note: This specification only applies to scrolling methods supported by the user agent; it does not require the user agent to support any particular input or scrolling method.

6.2. Choosing Snap Positions

A scroll container can have many snap areas scattered throughout its scrollable overflow area. A naïve algorithm for selecting a snap position can produce behavior that is unintuitive for users, so care is required when designing a selection algorithm. Here are a few pointers that can aid in the selection process:

Appendix A: Longhands

The physical and logical longhands (and their shorthands) interact as defined in [CSS-LOGICAL-1].

Physical Longhands for scroll-padding

Name: scroll-padding-top, scroll-padding-right, scroll-padding-bottom, scroll-padding-left
Value: auto | <length-percentage>
Initial: auto
Applies to: scroll containers
Inherited: no
Percentages: relative to the scroll container’s scrollport
Computed value: the keyword auto or a computed <length-percentage> value
Canonical order: per grammar
Animation type: by computed value type

These longhands of scroll-padding specify the top, right, bottom, and left edges of the snapport, respectively. Negative values are invalid.

Flow-relative Longhands for scroll-padding

Name: scroll-padding-inline-start, scroll-padding-block-start, scroll-padding-inline-end, scroll-padding-block-end
Value: auto | <length-percentage>
Initial: auto
Applies to: scroll containers
Inherited: no
Percentages: relative to the scroll container’s scrollport
Computed value: the keyword auto or a computed <length-percentage> value
Canonical order: per grammar
Animation type: by computed value type

These longhands of scroll-padding specify the block-start, inline-start, block-end, and inline-end edges of the snapport, respectively. Negative values are invalid.

Name: scroll-padding-block, scroll-padding-inline
Value: [ auto | <length-percentage> ]{1,2}
Initial: auto
Applies to: scroll containers
Inherited: no
Percentages: relative to the scroll container’s scrollport
Computed value: see individual properties
Animation type: by computed value
Canonical order: per grammar

These shorthands of scroll-padding-block-start + scroll-padding-block-end and scroll-padding-inline-start + scroll-padding-inline-end are longhands of scroll-padding, and specify the block-axis and inline-axis edges of the snapport, respectively.

If two values are specified, the first gives the start value and the second gives the end value.

Physical Longhands for scroll-margin

Name: scroll-margin-top, scroll-margin-right, scroll-margin-bottom, scroll-margin-left
Value: <length>
Initial: 0
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: absolute length
Canonical order: per grammar
Animation type: by computed value type

These longhands of scroll-margin specify the top, right, bottom, and left edges of the scroll snap area, respectively.

Flow-relative Longhands for scroll-margin

Name: scroll-margin-block-start, scroll-margin-inline-start, scroll-margin-block-end, scroll-margin-inline-end
Value: <length>
Initial: 0
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: absolute length
Canonical order: per grammar
Animation type: by computed value type

These longhands of scroll-margin specify the block-start, inline-start, block-end, and inline-end edges of the scroll snap area, respectively.

Name: scroll-margin-block, scroll-margin-inline
Value: <length>{1,2}
Initial: 0
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: see individual properties
Animation type: by computed value type
Canonical order: per grammar

These shorthands of scroll-margin-block-start + scroll-margin-block-end and scroll-margin-inline-start + scroll-margin-inline-end are longhands of scroll-margin, and specify the block-axis and inline-axis edges of the scroll snap area, respectively.

If two values are specified, the first gives the start value and the second gives the end value.

7. Privacy and Security Considerations

This specification does not expose any information whatsoever that is not already exposed to the DOM directly; it just makes scrolling slightly more functional. There are no new privacy or security considerations.

8. Acknowledgements

Many thanks to David Baron, Simon Fraser, Håkon Wium Lie, Theresa O’Connor, François Remy, Majid Valpour, and most especially Robert O’Callahan for their proposals and recommendations, which have been incorporated into this document.

9. Changes

9.1. Changes Since 19 March 2019 CR

Changes since the 19 March 2019 Candidate Recommendation include:

A Disposition of Comments is available.

9.2. Changes Since 31 January 2019 CR

Changes since the 31 January 2019 Candidate Recommendation include:

9.3. Changes Since 14 August 2018 CR

Changes since the 14 August 2018 Candidate Recommendation include:

A Disposition of Comments is available.

9.4. Changes Since 14 December 2017 CR

Changes since the 14 December 2017 Candidate Recommendation include:

A Disposition of Comments is available.

9.5. Changes Since 24 August 2017 CR

Changes since the 24 August 2017 Candidate Recommendation include:

A Disposition of Comments is available.

9.6. Changes Since 20 October 2016 CR

Changes since the 20 October 2016 Candidate Recommendation include:

A Disposition of Comments is available.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

CR exit criteria

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

independent
each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.
interoperable
passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.
implementation
a user agent which:
  1. implements the specification.
  2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or "nightly build"). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
  3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-ALIGN-3]
Elika Etemad; Tab Atkins Jr.. CSS Box Alignment Module Level 3. 21 April 2020. WD. URL: https://www.w3.org/TR/css-align-3/
[CSS-BOX-4]
Elika Etemad. CSS Box Model Module Level 4. 21 April 2020. WD. URL: https://www.w3.org/TR/css-box-4/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. 19 January 2021. WD. URL: https://www.w3.org/TR/css-cascade-5/
[CSS-DISPLAY-3]
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 18 December 2020. CR. URL: https://www.w3.org/TR/css-display-3/
[CSS-LOGICAL-1]
Rossen Atanassov; Elika Etemad. CSS Logical Properties and Values Level 1. 27 August 2018. WD. URL: https://www.w3.org/TR/css-logical-1/
[CSS-OVERFLOW-3]
David Baron; Elika Etemad; Florian Rivoal. CSS Overflow Module Level 3. 3 June 2020. WD. URL: https://www.w3.org/TR/css-overflow-3/
[CSS-POSITION-3]
Elika Etemad; et al. CSS Positioned Layout Module Level 3. 19 May 2020. WD. URL: https://www.w3.org/TR/css-position-3/
[CSS-PSEUDO-4]
Daniel Glazman; Elika Etemad; Alan Stearns. CSS Pseudo-Elements Module Level 4. 31 December 2020. WD. URL: https://www.w3.org/TR/css-pseudo-4/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 6 June 2019. CR. URL: https://www.w3.org/TR/css-values-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 11 November 2020. WD. URL: https://www.w3.org/TR/css-values-4/
[CSS-WRITING-MODES-4]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 4. 30 July 2019. CR. URL: https://www.w3.org/TR/css-writing-modes-4/
[CSS2]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS21/
[CSSOM-VIEW-1]
Simon Pieters. CSSOM View Module. 17 March 2016. WD. URL: https://www.w3.org/TR/cssom-view-1/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[SELECTORS-4]
Elika Etemad; Tab Atkins Jr.. Selectors Level 4. 21 November 2018. WD. URL: https://www.w3.org/TR/selectors-4/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
scroll-margin <length>{1,4} 0 all elements no n/a by computed value type per grammar per side, an absolute length
scroll-margin-block <length>{1,2} 0 all elements no n/a by computed value type per grammar see individual properties
scroll-margin-block-end <length> 0 all elements no n/a by computed value type per grammar absolute length
scroll-margin-block-start <length> 0 all elements no n/a by computed value type per grammar absolute length
scroll-margin-bottom <length> 0 all elements no n/a by computed value type per grammar absolute length
scroll-margin-inline <length>{1,2} 0 all elements no n/a by computed value type per grammar see individual properties
scroll-margin-inline-end <length> 0 all elements no n/a by computed value type per grammar absolute length
scroll-margin-inline-start <length> 0 all elements no n/a by computed value type per grammar absolute length
scroll-margin-left <length> 0 all elements no n/a by computed value type per grammar absolute length
scroll-margin-right <length> 0 all elements no n/a by computed value type per grammar absolute length
scroll-margin-top <length> 0 all elements no n/a by computed value type per grammar absolute length
scroll-padding [ auto | <length-percentage> ]{1,4} auto scroll containers no relative to the corresponding dimension of the scroll container’s scrollport by computed value type per grammar per side, either the keyword auto or a computed <length-percentage> value
scroll-padding-block [ auto | <length-percentage> ]{1,2} auto scroll containers no relative to the scroll container’s scrollport by computed value per grammar see individual properties
scroll-padding-block-end auto | <length-percentage> auto scroll containers no relative to the scroll container’s scrollport by computed value type per grammar the keyword auto or a computed <length-percentage> value
scroll-padding-block-start auto | <length-percentage> auto scroll containers no relative to the scroll container’s scrollport by computed value type per grammar the keyword auto or a computed <length-percentage> value
scroll-padding-bottom auto | <length-percentage> auto scroll containers no relative to the scroll container’s scrollport by computed value type per grammar the keyword auto or a computed <length-percentage> value
scroll-padding-inline [ auto | <length-percentage> ]{1,2} auto scroll containers no relative to the scroll container’s scrollport by computed value per grammar see individual properties
scroll-padding-inline-end auto | <length-percentage> auto scroll containers no relative to the scroll container’s scrollport by computed value type per grammar the keyword auto or a computed <length-percentage> value
scroll-padding-inline-start auto | <length-percentage> auto scroll containers no relative to the scroll container’s scrollport by computed value type per grammar the keyword auto or a computed <length-percentage> value
scroll-padding-left auto | <length-percentage> auto scroll containers no relative to the scroll container’s scrollport by computed value type per grammar the keyword auto or a computed <length-percentage> value
scroll-padding-right auto | <length-percentage> auto scroll containers no relative to the scroll container’s scrollport by computed value type per grammar the keyword auto or a computed <length-percentage> value
scroll-padding-top auto | <length-percentage> auto scroll containers no relative to the scroll container’s scrollport by computed value type per grammar the keyword auto or a computed <length-percentage> value
scroll-snap-align [ none | start | end | center ]{1,2} none all elements no n/a discrete per grammar two keywords
scroll-snap-stop normal | always normal all elements no n/a discrete per grammar specified keyword
scroll-snap-type none | [ x | y | block | inline | both ] [ mandatory | proximity ]? none all elements no n/a discrete per grammar specified keyword(s)