code: function makeUrlsRelativeToCss(match, quotationMark, url) {
              if (PathUtils.isRelativeUrl(url)) {
                  var absUrl = PathUtils.makeUrlAbsolute(url, docUrl);
                  return "url(" + quotationMark + absUrl + quotationMark + ")";
              }
              return match;
          }
docs: Determines whether an url() line contains a relative or absolute URL, and makes the URL absolute to the CSS file if it is relative