When I opened this link on PC browser http://www.w3schools.com/html/tryit.asp?filename=tryhtml5_video
The video displayed a preview image and I could see the loading progress bar is running, all worked fine.
But when I opened the link on mobile browser, the video totally empty without any thumbnail, and progress bar stopped. (plays OK)
Then I add the attribute "preload=auto" inside the video tag, then also no difference.
Here is my code.
<!DOCTYPE html>
<html>
<body>
<video preload="auto" width="400" controls>
<source src="mov_bbb.mp4" type="video/mp4">
<source src="mov_bbb.ogg" type="video/ogg">
Your browser does not support HTML5 video.
</video>
<p>
Video courtesy of
<a href="http://www.bigbuckbunny.org/" target="_blank">Big Buck Bunny</a>.
</p>
</body>
</html>
My device: Android 5.1.1/Chrome 48.0.2564.95