The xml:base attribute
<span ="">...</span>
The HTML id attribute serves the same purpose as the xml:id attribute and should be used instead.
<span id="elementId-1">...</span>
The HTML lang attribute serves the same purpose as the xml:lang attribute and should be used instead. However, the prefixed xml:lang attribute is sometimes used on the html element.
<dt lang="fr-CA">de rigeur</dt><html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
The xml:space attribute determines whether any white space in the content of the element or its descendants is to be preserved.
<span ="">...</span>
The xmlns attribute sets the default namespace URI for the current element and any of its descendants.
<special xmlns="http://ExampleOnly.com/namespace/"> <p xmlns="http://www.w3.org/1999/xhtml">This paragraph is truly HTML, but its parent is "special". </p></special>
Although it may be coded at any level, it typically appears only on the top-level html element. The value of the xmlns attribute for HTML tags is http://www.w3.org/1999/xhtml the same as for the 2000 W3C standard HTML.
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE html><html xmlns="http://www.w3.org/1999/xhtml"> ...</html>
Using xmlns as a prefix associates a namespace URI with the user-specified namespace prefix following the xmlns: part of the attribute name.
<special xmlns="http://ExampleOnly.com/namespace/" xmlns:html="http://www.w3.org/1999/xhtml"> <html:p>This paragraph is truly HTML, but its parent is "special". </html:p></special>
You typically would only see something like this when HTML is aggregated with non-HTML content with element names from a different namespace.