function cssParseUri(candidate) {
      var string1 = /^\s*["]([^"]*)["]\s*$/;
      var string2 = /^\s*[']([^']*)[']\s*$/;
      var url1 = /^\s*url\s*[(]["]([^"]*)["][)]\s*$/;
      var url2 = /^\s*url\s*[(][']([^']*)['][)]\s*$/;
      // Not officially part of the CSS2.1 grammar
      // but supported by Chrome
      var url3 = /^\s*url\s*[(]([^)]*)[)]\s*$/;
      var match;
      if ((match = string1.exec(candidate))) {
        return match[1];
      } else if ((match = string2.exec(candidate))) {
        return match[1];
      } else if ((match = url1.exec(candidate))) {
        return match[1];
      } else if ((match = url2.exec(candidate))) {
        return match[1];
      } else if ((match = url3.exec(candidate))) {
        return match[1];
      }
      return null;
    }
Extracts a url out of an at-import rule of the form:
  \@import "mystyle.css";
  \@import url("mystyle.css");

Returns null if no valid url was found.