namespaces - is the xsi: prefix assumed to be known in XML?

Question

Working on an XML parser, I see a lot of XML files using the xsi:schemaLocation attribute without declaring the xsi namespace.

Is it an error or is there a spec stating it is assumed to be known before parsing? Grepping the namespaces spec for "xsi" gives no results, but maybe the answer is in another document.

Answer 1

No namespaces [other than the XML namespace] can be assumed to be known.

If a document contains an xsi prefix, there should also be an associated namespace (which would normally be xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance").

If the namespace is not declared, the XML file is not valid.