docs: function(url, callback) {
      var link = document.createElement('link');
      link.type = 'text/css';
      link.rel = 'stylesheet';
      if (useOnload)
        link.onload = function() {
          link.onload = function() {};
          // for style dimensions queries, a short delay can still be necessary
          setTimeout(callback, 7);
        }
      else
        var loadInterval = setInterval(function() {
          for (var i = 0; i < document.styleSheets.length; i++) {
            var sheet = document.styleSheets[i];
            if (sheet.href == link.href) {
              clearInterval(loadInterval);
              return callback();
            }
          }
        }, 10);
      link.href = url;
      head.appendChild(link);
    }