XML namespace
The reserved xml: namespace
Bound to the xml: prefix implicitly and permanently, and the source of xml:lang, xml:space, xml:base and xml:id. Never declare it, never rebind it.
xmlns:xml="http://www.w3.org/XML/1998/namespace"The xml: prefix is bound to this URI by the XML specification itself. It needs no xmlns:xml declaration, and Namespaces in XML §2.3 forbids binding the prefix to any other URI.
Forbidden is not the same as caught, and this is a case where the two come apart. libxml2 — the parser behind this site, and behind PHP, lxml and Nokogiri — accepts a document that rebinds xml: to a different URI without a word of complaint. The rule is real, nothing enforces it for you, and a document relying on that silence will break the day it meets a parser that does check.
It supplies the four attributes every XML vocabulary can use: xml:lang, xml:space, xml:base and xml:id.
- URI
http://www.w3.org/XML/1998/namespace- Prefix
xml- Elements
- Contributes attributes only
Specification: Namespaces in XML 1.0
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