CSS namespaces

CSS namespaces are used to restrict selectors to elements on a certain XML namespace. For an overview of CSS namespaces, see @namespace on MDN. A namespace in a user style is not the same as a namespace in a user script. Do not make up your own URL; this can break your style.

Specifying a CSS namespace in a user style is generally not required as long as the style is completely specific to a certain site. It can be included anyway, if you wish, with the following code at the top of your style:

@namespace url(http://www.w3.org/1999/xhtml);

(Do not modify the URL.)

If the user style is supposed to apply globally (i.e. to all web sites), include the same code at the top of your style:

@namespace url(http://www.w3.org/1999/xhtml);

If the user style is supposed to apply to Firefox's user interface, include this at the top of your style:

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

If you know what you're doing, feel free to use other namespaces or to have multiple ones.