[Doitsu]

@Gregg Bell:

AFAIK, CSS namespace declarations are a CSS3-only feature and will always generate W3C error messages if the selected Stylesheet Validation Level is CSS Level 2.1.

You could theoretically change the CSS level via Edit > Preferences > Settings > General Settings, and if you change the value to CSS Level 3, the W3C validator will no longer display a namespace error message. However, selecting CSS Level 3 isn't recommended for epub2 book stylesheets, because the epub2 standard was based on CSS Level 2.1 and some older epub engines might not support CSS3-only elements.

Therefore it's better to simply delete CSS namespace declarations in epub2 books, unless the (X)HTML files contain non-standard namespace declarations.

AFAIK, CSS namespace declarations are only required for epub3 book stylesheets and only if you want to select elements with epub:type attributes.

For example the following stylesheet code:

@namespace epub "http://www.idpf.org/2007/ops";

*[epub|type='pagebreak'] { display: none; }

will hide epub3 page number targets formatted as:

<span epub:type="pagebreak" id="page1">1</span>