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The prefixed attributes, such as xml:lang="", are "namespaced" within XHTML, SVG and MathML. Thus, they can be styled via CSS3 namespaces. http://www.w3.org/TR/css3-namespace/ As for the HTML serialization, then xml:lang does not have the xml namespace effect. This means that a style such as <style type="text/css"> @namespace xml "http://www.w3.org/XML/1998/namespace"; *[xml|lang]{background:lime;} </style> would work XHTML, SVG and Math, it would not work in HTML. While authors might seldom use this way of styling, the spec should explain whether it can be used.
Added section: 7.3.2 Attributes Styled Using CSS Namespaces The prefixed attributes, such as xml:lang="", are "namespaced" within XHTML, SVG and MathML. Thus, they can be styled via CSS3 namespaces. [CSS3NAMESPACE] However, for the HTML serialization, xml:lang would then not have the xml namespace effect. A style such as the following is valid in XHTML, SVG, and MathML, it does not work in HTML and is therefore not used in polyglot markup. Example 11 <style type="text/css"> @namespace xml "http://www.w3.org/XML/1998/namespace"; *[xml|lang]{background:lime;} </style> new revision: 1.101; previous revision: 1.100