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Editorial Team
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Asked: 2026-05-18T05:10:50+00:00

I was wondering what values are allowed for the xml:base attribute?

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I was wondering what values are allowed for the xml:base attribute?

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  1. Editorial Team

    See XML Base

    The attribute xml:base may be inserted in XML documents to specify a base URI other than the base URI of the document or external entity. The value of this attribute is interpreted as a Legacy Extended IRI (LEIRI) as defined in the W3C Note "Legacy extended IRIs for XML resource identification"

    See Legacy extended IRIs for XML resource identification for technical details.

    Here’s an example from the spec:

    <?xml version="1.0"?>
<doc xml:base="http://example.org/today/"
     xmlns:xlink="http://www.w3.org/1999/xlink">
  <head>
    <title>Virtual Library</title>
  </head>
  <body>
    <paragraph>See <link xlink:type="simple" xlink:href="new.xml">what's
      new</link>!</paragraph>
    <paragraph>Check out the hot picks of the day!</paragraph>
    <olist xml:base="/hotpicks/">
      <item>
        <link xlink:type="simple" xlink:href="pick1.xml">Hot Pick #1</link>
      </item>
      <item>
        <link xlink:type="simple" xlink:href="pick2.xml">Hot Pick #2</link>
      </item>
      <item>
        <link xlink:type="simple" xlink:href="pick3.xml">Hot Pick #3</link>
      </item>
    </olist>
  </body>
</doc>

    Edit: IRI is an extension of URI, which is an extension of URL; so for the simple scenarios, you could think of them as URLs with lots of extensions.

    RFC3987:

    A Uniform Resource Identifier (URI) is defined in [RFC3986] as a
    sequence of characters chosen from a limited subset of the repertoire
    of US-ASCII [ASCII] characters.
    This document defines a new protocol element called Internationalized
    Resource Identifier (IRI) by extending the syntax of URIs to a much
    wider repertoire of characters.

    According to the spec,

    The syntax of Legacy Extended IRIs (LEIRIs) and LEIRI references is the same as that for IRIs and IRI references except that ucschar is redefined.

    IRI:

    ucschar        ::= %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
                   / %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
                   / %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
                   / %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
                   / %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
                   / %xD0000-DFFFD / %xE1000-EFFFD

    LEIRI:

    ucschar    ::=  " " / "<" / ">" / '"' / "{" / "}" / "|"
                    / "\" / "^" / "`" / %x0-1F / %x7F-D7FF
                    / %xE000-FFFD / %x10000-10FFFF
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