code: function initVideoFallback(opt_config) {
    var videoSelector = (opt_config && opt_config.videoSelector)
        || defaultConfig.fallbackVideoSelector;
    var imageSelector = (opt_config && opt_config.imageSelector)
        || defaultConfig.fallbackImageSelector;
    var breakpoint = (opt_config && opt_config.breakpoint)
        || defaultConfig.breakpoint;
  
    var hidden = {'display': 'none !important'};
    var rules = {};
    if (canPlayVideo()) {
      rules[imageSelector] = hidden;
    } else {
      rules[videoSelector] = hidden;
    }
  
    if (!breakpoint) {
      ui.createStyle(rules);
    } else {
      var minScreenQuery = '(min-width: ' + breakpoint + 'px)';
      ui.createStyle(rules, minScreenQuery);
      var smallerBreakpoint = breakpoint - 1;
      var maxScreenQuery = '(max-width: ' + smallerBreakpoint + 'px)';
      rules = {};
      rules[videoSelector] = hidden;
      ui.createStyle(rules, maxScreenQuery);
    }
  }
docs: Shows or hides video elements and their corresponding image elements
  depending on whether the user agent can play video.
  
  Fallback is only implemented between video and images, not video formats.
  You should rely on the video element's default behavior with <source> tags
  to support fallback between video formats. As a result, you should probably
  always include at least a mp4 video, as mp4 is the most compatible.
  
  This is opinionated and will not enable video for mobile devices.
  
  (1) Set up a DOM structure:
  
  <div class="ak-video-fallback">
  <video>...</video>
  <img>
  </div>
  
  (2) Run at the top of the page (to avoid FOUC):
  
  initVideoFallback();
  
  @param {Object=} opt_config Configuration. The configuration contains two
  options: videoSelector and imageSelector, selectors used to query
  videos and images to hide.