As explained in this article Google recently changed Chrome so that by default it will no longer autoplay video unless certain conditions are true.
Web-browsers being run on mobile devices generally don't allow autoplay of media files like audio & video, are you doing your testing on a mobile device?
I suggests you look at the video element documentation because there are also a number of issues in your example,
1. The preload attribute needs to be assigned one of a known set of values, "preload" isn't one of them, and as explained in the documentation including a autoplay attribute automatically makes this attribute redundant.
2. The autoplay and loop attributes are Boolean in nature, so you don't need to assign then a value.
3. The source element is self ending, so there is no need for a end source tag.
4. Line-breaks within the source area of the video element should be escaped.
5. [optional] Ideally you should include a text message for those running a web-browser that doesn't support the video element.
The following is a modified version of your example which should work as you want within web-browsers that support autoplay and loop, and under the conditions that that specific web-browser requires.
<video id="video" autoplay loop>\
<source src="media/example.webm" type="video/webm">\
<p>Your browser doesn't support HTML5 video.</p>\
<video id="video" autoplay loop>
<source src="media/example.webm" type="video/webm">
<p>Your browser doesn't support HTML5 video.</p>
edit: added an example that uses Collapsing whitespace markup