How to stop autoplaying HTML5 videos

P. Petropoulos blocked Flash videos from playing automatically without his permission. Now he wants to block HTML5 videos, as well.
On New Year’s Day, I told you how to block videos that play automatically when you visit a Web page. In fact, if such a video is starting to play right about…now, you may want to revisit that article.

My previous instructions concentrated on Flash videos, however, and Flash isn’t the only technology designed to annoy you with unwanted videos. So now I’ll tell you how to block HTML5 videos in Chrome and Firefox.
[Have a tech question? Ask PCWorld Contributing Editor Lincoln Spector. Send your query to answer@pcworld.com.]
Alas, I could find no solution for Internet Explorer. If you find one, just put it in the Comments section below.

Chrome

You need to download and install a Chrome extension called, simply enough, Stop YouTube HTML5 Autoplay, by Port Zero. (If that link fails, try the developer’s site and click the link provided there.) And yes, it blocks non-Youtube HTML5 videos as well.
This program, to use a marketing term for one major tech company, “just works.” Install and enable it, and HTML5 videos won’t start automatically. You can click the video to get it going.

Firefox

You don’t need an add-on or an extension. You can take care of this with a setting in Firefox itself.
Open a new Firefox window or tab, and in the address field (where you would normally type a URL), enter about:config. Then Click the scary I’ll be careful, I promise button.

You’ll get a long list of settings, and above them all, a search field. In that search field, type autoplay and press Enter. This will bring up one entry: media.autoplay.enabled.
Note the Value column. It will probably be True. Double-click the entry and the Value will change to False.

The next time you visit a page with an HTML5 autoplaying video, the video box will appear with an image in it. But the video won’t start until you tell it to.

Source: http://www.pcworld.com/article/2922403/how-to-stop-autoplaying-html5-videos.html#tk.rss_howto