What does @-moz-document url-prefix() do?

Question

In Simon Collison's new Responsive Web Design, in the CSS, there are several declarations like this:

@-moz-document url-prefix() {
  .fl { float:left; margin:12px 4px 0 0; padding:0; font-size:65px;  line-height:62%;  color:#ba1820; }
  .fs { float:left; margin:12px 4px 10px 0; padding:0; font-size:65px; line-height:62%; color:#ba1820; }
}

What does this actually do? I've googled for @-moz-document url-prefix() and have found references for its use within userchrome but not standard site stylesheets.

It usually has a URL passed passed in as an argument which then restricts the content of the declaration to that URL. However, on Colly's site, there is no argument being passed in. This would indicate that the declaration is operating on the current URL, or any URL, no? So is what we're seeing here a way of targeting Mozilla-only browsers with certain rules?

Answer 1

Any CSS rule that starts with @-moz- is a Gecko layout engine specific rule, not a standard rule. That is, it is a mozilla specific extension.

The url-prefix rule applies the rule to any page whose URL starts with it.

See here for a list of mozilla specific extensions.

Answer 2

From https://developer.mozilla.org/en/CSS/@-moz-document

       @-moz-document url(http://www.w3.org/),
                   url-prefix(http://www.w3.org/Style/),
                   domain(mozilla.org)
    {
      /* CSS rules here apply to:
         + The page "http://www.w3.org/".
         + Any page whose URL begins with "http://www.w3.org/Style/"
         + Any page whose URL's host is "mozilla.org" or ends with
           ".mozilla.org"
       */

      /* make the above-mentioned pages really ugly */
      body { color: purple; background: yellow; }
}