<base>

<base> sets the document’s base URL for relative URLs, a default navigation target, or both. It belongs in head before elements with URL-valued attributes. Use at most one base element.

It must have href, target, or both. href alone is not required if target is present.

Inspect a complete document

<a id="lesson" href="layout.html">Layout lesson</a>
<p id="resolved"></p>
<script>
  document.querySelector('#resolved').textContent =
    document.querySelector('#lesson').href;
</script>

The displayed address is https://example.com/guides/layout.html. JavaScript only exposes the resolved address; the base URL itself is native HTML behavior. Inspect the anchor in developer tools or copy the document to compare its literal href with its resolved URL.

A practical use

A generated document that lives under a known root can resolve all its relative asset URLs from that root. Check every relative URL before adding a base: a stylesheet, image, form action, and fragment link can all change meaning.

Common mistakes

href="#section" also resolves against the base URL, so it can leave the current document. Multiple base elements are invalid authoring: browsers use the first href and first target. data: and javascript: base URLs are not allowed.

target="_blank" sets a default browsing target for links and forms that do not choose one themselves. Opening new tabs can surprise readers; use it only when the interface calls for that behavior. This site’s preview cannot open a new browsing context.

Sources and browser support

MDN: base describes the syntax and links to the specification and compatibility data. Check individual values in that table when supporting older browsers.