amp-story video not displaying full picture (width, height issues)

问题

the desired behavior

• AMP allows you to embed images, video and audio content.
• The full width of the content should be viewable by users

a specific problem or error

• It is possible to upload a video in amp-story and not see the full width

the shortest code necessary to reproduce it in the question itself

    <amp-story-page id="cover page" auto-advance-after="cover" >
         <amp-story-grid-layer template="fill">
              <amp-video placeholder="img/icon.png"
                         id="cover"
                         controls 
                         autoplay
                         width="720"
                         height= "1280"
                         layout="fill" />
              <source src="video/cat.mp4"
              type="video/mp4" />
              <div fallback>
              <p>This browser does not support the video element.</p>
              </div>
             </amp-video>
        </amp-story-grid-layer>
    </amp-story-page> 

• I am using amp-story
• I uploaded my video to public/video/cat.mp4

来源：https://stackoverflow.com/questions/52727981/amp-story-video-not-displaying-full-picture-width-height-issues