title: What does @-moz-document url-prefix() do?
body: <p>In Simon Collison's <strike>new</strike> <a href="https://colly.com/v4/" rel="noreferrer">old Responsive Web Design</a>, in the CSS, there are several declarations like this:</p>
  
  <pre><code>@-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; }
  }
  </code></pre>
  
  <p>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. </p>
  
  <p>It usually has a URL 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 <em>any</em> URL, no? So <strong>is what we're seeing here a way of targeting Mozilla-only browsers with certain rules</strong>?</p>