base

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base

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W3C Recommendation

Summary

The base element is used to specify a document's base URL and base target that is used for resolving relative URLs within the document.


Overview Table

DOM Interface HTMLBaseElement
Permitted contents none
Permitted parents Only permitted to occur once within <head>.
Tag omission A <base> element must not have an end tag.

A relative URL (some/example.html) needs to be transformed to a fully qualified URL (http://example.org/some/example.html) before it can be downloaded. Usually the document's URL (available to JavaScript through the location object) is used as the base URL for resolving relative URLs. The <base> element allows you to override this default with the href attribute.

Links (<a>) and forms (<form>) open in a (target). The default target is _self, resulting in the link opening in the same window as the document currently viewed. This default can be overridden document-wide using <base target="…">.

If a document is integrated in an iframe, it may help to specify <base target="_parent"> in order to open the links within the iframe in the scope parent document. If _parent or _top are used without the document really being integrated in an hierarchy, expect the behavior of _self.


Compatibility

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Desktop

FeaturesChromeFirefoxInternet ExplorerOperaSafari
Basic support
?
Yes
1.0
Yes
?
Yes
?
Yes
?
Yes

Mobile

FeaturesAndroidFirefox MobileIE MobileOpera MobileSafari Mobile
Basic support
?
Yes
1.0
Yes
?
Yes
?
Yes
?
Yes

Examples

The example shows a link with the relative destination some-file.html that gets rewritten to http://example.org/deep/some-file.html

HTML

<!DOCTYPE html>
<html>
  <head>
    <title>base element example</title>
    <base href="http://www.example.org/deep/">
  </head>
  <body>
    <p>A <a href="some-file.html">relative link</a>.</p>
    <!-- after resolving the above link equals to -->
    <p>A <a href="http://www.example.org/deep/some-file.html">relative link</a>.</p>
  </body>
</html>

The example shows that base only affects elements following it

HTML

<head>
  <title>base element example</title>
  <link href="my-style.css" rel="stylesheet">
  <base href="http://example.com">
  <link href="my-other-style.css" rel="stylesheet">
  <!--
    resolves to
    [current domain and directory]/my-style.css
    http://example.com/my-other-style.css
  -->
</head>

The example shows how multiple base occurrences are collapsed and ignored

HTML

<head>
  <title>base element example</title>
  <base href="http://example.com">
  <base target="_blank">
  <base href="http://webplatform.org" target="_top">
  <!--
    equals to the single definition:
    <base href="http://example.com/" target="_blank">
    except for Internet Explorer, where only the first element is read:
    <base href="http://example.com/" target="_self">    
  -->
</head>

Notes

  • Relative URLs within <base> don't work in every browser, resolving a relative base URL was introduced in Firefox 4 and Internet Explorer 10.
  • <base> only affects elements following it's declaration.
  • multiple <base> declarations are illegal, only the first href and target are used, the rest is discarded. Internet Explorer ignores all <base> instances after the first.

Note Inline SVGs using references like fill="url(#element-id)" can be a problem in documents using <base>. The reason is that url(#element-id) is actually a URL, not a CSS selector. At least Firefox and Chrome are susceptible to this behavior.


Related specifications

Specification Status Related Changes
HTML 4.01 Specification W3C Recommendation
HTML5 W3C Working Draft
WHATWG Living Standard

See also

External resources