Here is an example of the code:
<video controls autoplay>
<source src="kacnje.mp4" type="video/mp4">
<source src="kacnje.ogg" type="video/ogg">
Sorry - your browser does not support the video tag.
How can user fights against website with video autoplay?
1. Mozilla Firefox
In the address field (where you normally type URL), enter about:config and press Enter. A warning appears and you have to press the scary button with "I'll be careful, I promise" label.
You will get a list of settings with a search field. In that search field, type autoplay and press Enter. This will find only one entry - media.autoplay.enabled. It will probably be set to True. As you expect, use double click to change the value to False.
The next time you visit a webpage with HTML5 autoplay video, the video box will appear, but the video won't start without your intervention.
2. Google Chrome
Chrome doesn't have this feature built-in. It is possible to prevent HTML5 videos to autoplay by installing the browser extension from Chrome Web Store. I can recommend Stop YouTube HTML5 AutoPlay (it should works with all websites, not just with YouTube).