HTML5

To parse a URL url into its component parts, the user agent must use the parse an address algorithm defined by the IRI specification. [RFC3987]

Parsing a URL can fail. If it does not, then it results in the following components, again as defined by the IRI specification:

• <scheme>
• <host>
• <port>
• <hostport>
• <path>
• <query>
• <fragment>
• <host-specific>

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To resolve a URL to an absolute URL relative to either another absolute URL or an element, the user agent must use the following steps. Resolving a URL can result in an error, in which case the URL is not resolvable.

1. Let url be the URL being resolved.

2. Let encoding be determined as follows:

   If the URL had a character encoding defined when the URL was created or defined

   The URL character encoding is as defined.

   If the URL came from a script (e.g. as an argument to a method)

   The URL character encoding is the script's URL character encoding.

   If the URL came from a DOM node (e.g. from an element)

   The node has a Document, and the URL character encoding is the document's character encoding.

3. If encoding is a UTF-16 encoding, then change the value of encoding to UTF-8.

4. If the algorithm was invoked with an absolute URL to use as the base URL, let base be that absolute URL.

   Otherwise, let base be the base URI of the element, as defined by the XML Base specification, with the base URI of the document entity being defined as the document base URL of the Document that owns the element. [XMLBASE]

   For the purposes of the XML Base specification, user agents must act as if all Document objects represented XML documents.

   It is possible for xml:base attributes to be present even in HTML fragments, as such attributes can be added dynamically using script. (Such scripts would not be conforming, however, as xml:base attributes are not allowed in HTML documents.)

   The document base URL of a Document object is the absolute URL obtained by running these substeps:

   1. Let fallback base url be the document's address.

   2. If fallback base url is about:blank, and the Document's browsing context has a creator browsing context, then let fallback base url be the document base URL of the creator Document instead.

   3. If there is no base element that is both a child of the head element and has an href attribute, then the document base URL is fallback base url.

   4. Otherwise, let url be the value of the href attribute of the first such element.

   5. Resolve url relative to fallback base url (thus, the base href attribute isn't affected by xml:base attributes).

   6. The document base URL is the result of the previous step if it was successful; otherwise it is fallback base url.

5. Return the result of applying the resolve an address algorithm defined by the IRI specification to resolve url relative to base using encoding encoding. [RFC3987]

2.5.2 Dynamic changes to base URLs

Status: Last call for comments

When an xml:base attribute changes, the attribute's element, and all descendant elements, are affected by a base URL change.

When a document's document base URL changes, all elements in that document are affected by a base URL change.

When an element is moved from one document to another, if the two documents have different base URLs, then that element and all its descendants are affected by a base URL change.

When an element is affected by a base URL change, it must act as described in the following list:

If the element is a hyperlink element

If the absolute URL identified by the hyperlink is being shown to the user, or if any data derived from that URL is affecting the display, then the href attribute should be re-resolved relative to the element and the UI updated appropriately.

For example, the CSS :link/:visited pseudo-classes might have been affected.

If the element is a q, blockquote, section, article, ins, or del element with a cite attribute

If the absolute URL identified by the cite attribute is being shown to the user, or if any data derived from that URL is affecting the display, then the URL should be re-resolved relative to the element and the UI updated appropriately.

Otherwise

The element is not directly affected.

Changing the base URL doesn't affect the image displayed by img elements, although subsequent accesses of the src IDL attribute from script will return a new absolute URL that might no longer correspond to the image being shown.

           attribute DOMString protocol;
           attribute DOMString host;
           attribute DOMString hostname;
           attribute DOMString port;
           attribute DOMString pathname;
           attribute DOMString search;
           attribute DOMString hash;

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The attributes defined to be URL decomposition IDL attributes must act as described for the attributes with the same corresponding names in this section.

In addition, an interface with a complement of URL decomposition IDL attributes will define an input, which is a URL that the attributes act on, and a common setter action, which is a set of steps invoked when any of the attributes' setters are invoked.

The seven URL decomposition IDL attributes have similar requirements.

On getting, if the input is an absolute URL that fulfills the condition given in the "getter condition" column corresponding to the attribute in the table below, the user agent must return the part of the input URL given in the "component" column, with any prefixes specified in the "prefix" column appropriately added to the start of the string and any suffixes specified in the "suffix" column appropriately added to the end of the string. Otherwise, the attribute must return the empty string.

On setting, the new value must first be mutated as described by the "setter preprocessor" column, then mutated by %-escaping any characters in the new value that are not valid in the relevant component as given by the "component" column. Then, if the input is an absolute URL and the resulting new value fulfills the condition given in the "setter condition" column, the user agent must make a new string output by replacing the component of the URL given by the "component" column in the input URL with the new value; otherwise, the user agent must let output be equal to the input. Finally, the user agent must invoke the common setter action with the value of output.

When replacing a component in the URL, if the component is part of an optional group in the URL syntax consisting of a character followed by the component, the component (including its prefix character) must be included even if the new value is the empty string.

The previous paragraph applies in particular to the ":" before a <port> component, the "?" before a <query> component, and the "#" before a <fragment> component.

For the purposes of the above definitions, URLs must be parsed using the URL parsing rules defined in this specification.

Attribute	Component	Getter Condition	Prefix	Suffix	Setter Preprocessor	Setter Condition
protocol	<scheme>	—	—	U+003A COLON (:)	Remove all trailing U+003A COLON characters (:)	The new value is not the empty string
host	<hostport>	input is an authority-based URL	—	—	—	The new value is not the empty string and input is an authority-based URL
hostname	<host>	input is an authority-based URL	—	—	Remove all leading U+002F SOLIDUS characters (/)	The new value is not the empty string and input is an authority-based URL
port	<port>	input is an authority-based URL, and contained a <port> component (possibly an empty one)	—	—	Remove all characters in the new value from the first that is not in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), if any. Remove any leading U+0030 DIGIT ZERO characters (0) in the new value. If the resulting string is empty, set it to a single U+0030 DIGIT ZERO character (0).	input is an authority-based URL, and the new value, when interpretted as a base-ten integer, is less than or equal to 65535
pathname	<path>	input is a hierarchical URL	—	—	If it has no leading U+002F SOLIDUS character (/), prepend a U+002F SOLIDUS character (/) to the new value	input is hierarchical
search	<query>	input is a hierarchical URL, and contained a <query> component (possibly an empty one)	U+003F QUESTION MARK (?)	—	Remove one leading U+003F QUESTION MARK character (?), if any	input is a hierarchical URL
hash	<fragment>	input contained a non-empty <fragment> component	U+0023 NUMBER SIGN (#)	—	Remove one leading U+0023 NUMBER SIGN character (#), if any	—