Is anyone actually using css namespaces?

Namespaces have a rather nasty syntax in CSS, because the ":" namespace character must be escaped by a leading backslash to differentiate it from a pseudo-class:

html\:img {
  border: 2px solid black;
}
html\:a:visited html\:img {
  border-color: grey;
}

This is only really useful when embedding HTML inside an XML document. When adding the html namespace, elements from the HTML namespace are correctly displayed as they would appear in HTML, allowed access to capabilities that are not yet provided by CSS.

<story xmlns:HTML="http://www.w3.org/Profiles/XHTML-transitional">
  ...
  <restaurant>
    <name>Red Apple Inn</name>
    <logo>
      <HTML:A href="javascript:alert('Visit the Red Apple Inn!')">
        <HTML:IMG src="red-apple.gif" height="50" width="200"/>
      </HTML:A>
    </logo>
    ...

In an HTML5 context, I can't think of any cases where you would need this. The only place where I've seen namespaces in CSS so far, is Webkit's default CSS for SVG or MathML, and they use a different syntax : the @namespace at-rule.

For example, this is code from WebKit/webkit/blob/master/Source/WebCore/css/mathml.css :

@namespace "http://www.w3.org/1998/Math/MathML";

math {
    -webkit-line-box-contain: glyphs replaced;
    text-indent: 0;
    direction: ltr;
}
mtext {
    line-height: 1.0;
}

...

This is code from WebKit/webkit/blob/master/Source/WebCore/css/svg.css :

@namespace "http://www.w3.org/2000/svg";
@namespace html "http://www.w3.org/1999/xhtml";

svg:not(:root), symbol, image, marker, pattern, foreignObject {
    overflow: hidden
}

svg:root {
    width: 100%;
    height: 100%
}

text, foreignObject {
    display: block
}

...

They cover completely different use cases.

CSS namespaces are for applying CSS to XML documents that mix elements from different XML namespaces. e.g. so you can target <foo:p> and <bar:p> without confusion.

SMACSS covers techniques for writing robust CSS that doesn't interfere with other parts of the page. e.g. so that .title in your address book HTML doesn't get muddled with .title in your list of publications HTML.

---

Further details from the spec:

Note: In HTML, the xmlns attribute has absolutely no effect. It is basically a talisman. It is allowed merely to make migration to and from XHTML mildly easier. When parsed by an HTML parser, the attribute ends up in no namespace, not the "http://www.w3.org/2000/xmlns/" namespace like namespace declaration attributes in XML do.