Visual effects

Generally, the content of a block box is confined to the content edges of the box. In certain cases, a box may overflow, meaning its content lies partly or entirely outside of the box, e.g.:

Whenever overflow occurs, the 'overflow' property specifies whether a box is clipped to its padding edge, and if so, whether a scrolling mechanism is provided to access any clipped out content.

11.1.1 Overflow: the 'overflow' property

This property specifies whether content of a block-level element is clipped when it overflows the element's box. It affects the clipping of all of the element's content except any descendant elements (and their respective content and descendants) whose containing block is the viewport or an ancestor of the element. Values have the following meanings:

Even if 'overflow' is set to 'visible', content may be clipped to a UA's document window by the native operating environment.

UAs must apply the 'overflow' property set on the root element to the viewport. HTML UAs must instead apply the 'overflow' property from the BODY element to the viewport, if the value on the HTML element is 'visible'. The 'visible' value when used for the viewport must be interpreted as 'auto'. The element from which the value is propagated must have a used value for 'overflow' of 'visible'.

In the case of a scrollbar being placed on an edge of the element's box, it should be inserted between the inner border edge and the outer padding edge. Any space taken up by the scrollbars should be subtracted from the computed width/height, thus preserving the inner border edge.

Consider the following example of a block quotation (<blockquote>) that is too big for its containing block (established by a <div>). Here is the ~~source~~ ~~document:~~source:


<div>
<blockquote>
<p>I didn't like the play, but then I saw
it under adverse conditions - the curtain was up.</p>
<cite>- Groucho Marx</cite>
</blockquote>
</div>

Here is the style sheet controlling the sizes and style of the generated boxes:


div { width : 100px; height: 100px;
      border: thin solid red;
      }

blockquote   { width : 125px; height : 100px;
      margin-top: 50px; margin-left: 50px; 
      border: thin dashed black
      }

cite { display: block;
       text-align : right; 
       border: none
       }

The initial value of 'overflow' is 'visible', so the <blockquote> would be formatted without clipping, something like this:

Rendered overflow [D]

Setting 'overflow' to 'hidden' for the <div>, on the other hand, causes the <blockquote> to be clipped by the containing block:

Clipped overflow [D]

A value of 'scroll' would tell UAs that support a visible scrolling mechanism to display one so that users could access the clipped content.

Finally, consider this case where an absolutely positioned element is mixed with an overflow parent.

Stylesheet:


  container { position: relative; border: solid; }
  scroller { overflow: scroll; height: 5em; margin: 5em; }
  satellite { position: absolute; top: 0; }
  body { height: 10em; }

Document fragment:


  <container>
   <scroller>
    <satellite/>
    <body/>
   </scroller>
  </container>

In this example, the "scroller" element will not scroll the "satellite" element, because the latter's containing block is outside the element whose overflow is being clipped and scrolled.

11.1.2 Clipping: the 'clip' property

A clipping region defines what portion of an element's border box is visible. By default, the element is not clipped. However, the clipping region may be explicitly set with the 'clip' property.

The 'clip' property applies only to absolutely positioned elements. Values have the following meanings:

An element's clipping region clips out any aspect of the element (e.g. content, children, background, borders, text decoration, outline and visible scrolling mechanism — if any) that is outside the clipping region.

The element's ancestors may also clip portions of their content (e.g. via their own 'clip' property and/or if their 'overflow' property is not 'visible'); what is rendered is the cumulative intersection.

If the clipping region exceeds the bounds of the UA's document window, content may be clipped to that window by the native operating environment.

The following two rules:


p { clip: rect(5px, 40px, 45px, 5px); }
p { clip: rect(5px, 55px, 45px, 5px); }

will create the rectangular clipping regions delimited by the dashed lines in the following illustrations:

Two clipping regions [D]

11.2 Visibility: the 'visibility' property

The 'visibility' property specifies whether the boxes generated by an element are rendered. Invisible boxes still affect layout (set the 'display' property to 'none' to suppress box generation altogether). Values have the following meanings:

This property may be used in conjunction with scripts to create dynamic effects.

In the following example, pressing either form button invokes an author-defined script function that causes the corresponding box to become visible and the other to be hidden. Since these boxes have the same size and position, the effect is that one replaces the other. (The script code is in a hypothetical script language. It may or may not have any effect in a CSS-capable UA.)


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML  4.01//EN" "http://www.w3.org/TR/1999/REC-html401-19991224/strict.dtd">4.01//EN">
<HTML>
<HEAD><TITLE>Dynamic visibility example</TITLE>
<META 
 http-equiv="Content-Script-Type"
 content="application/x-hypothetical-scripting-language">
<STYLE type="text/css">
<!--
   #container1 { position: absolute; 
                 top: 2in; left: 2in; width: 2in }
   #container2 { position: absolute; 
                 top: 2in; left: 2in; width: 2in;
                 visibility: hidden; }
-->
</STYLE>
</HEAD>
<BODY>
<P>Choose a suspect:</P>
<DIV id="container1">
   <IMG alt="Al Capone" 
        width="100" height="100" 
        src="suspect1.png">
   <P>Name: Al Capone</P>
   <P>Residence: Chicago</P>
</DIV>

<DIV id="container2">
   <IMG alt="Lucky Luciano" 
        width="100" height="100" 
        src="suspect2.png">
   <P>Name: Lucky Luciano</P>
   <P>Residence: New York</P>
</DIV>

<FORM method="post" 
      action="http://www.suspect.org/process-bums">
   <P>
   <INPUT name="Capone" type="button" 
          value="Capone" 
          onclick='show("container1");hide("container2")'>
   <INPUT name="Luciano" type="button" 
          value="Luciano" 
          onclick='show("container2");hide("container1")'>
</FORM>
</BODY>
</HTML>

Value:	visible \| hidden \| scroll \| auto \| inherit
Initial:	visible
Applies to:	non-replaced block-level elements, table cells, and inline-block elements
Inherited:	no
Percentages:	N/A
Media:	visual
Computed value:	as specified

Value:	<shape> \| auto \| inherit
Initial:	auto
Applies to:	absolutely positioned elements
Inherited:	no
Percentages:	N/A
Media:	visual
Computed value:	For rectangle values, a rectangle consisting of four computed lengths; otherwise, as specified

Value:	visible \| hidden \| collapse \| inherit
Initial:	visible
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	visual
Computed value:	as specified

11 Visual effects

11.1 Overflow and clipping

11.1.1 Overflow: the 'overflow' property

11.1.2 Clipping: the 'clip' property

11.2 Visibility: the 'visibility' property