Depends on the site. Firefox has context menus for saving html5 video, but they don't work if the site uses a customized player. For example, if you visit http://www.mozilla.com/en-US/firefox/video/, you will be able to right-click and select the Save video option.
On YouTube, that won't work. In this case, click the YouTube logo in the address bar, then click More information in the mini popup, then click the Media tab in the big popup dialog, then find the video and select it, then click Save As. Change the extension to webm before saving.
Alternatively, you can copy the address and load it on some download manager like DownThemAll or wget via terminal.
The Applications templates are not Ubuntu Touch templates, those are actually just Qt Creator templates, you can't use them for Ubuntu Apps.
When you make an Ubuntu HTML5 app you can use standard JavaScript mouse event handlers like onmousemove, onmouseover and onmouseout although right now those don't work very well. Ubuntu Touch is in rapid development though so keep an eye out for updates.
EDIT: Use touch event handlers as specified in the W3C Specification
You can't really quit a HTML5 app from the app itself. It's not really a problem since if you had a chance to play with other apps for Ubuntu Touch you might have noticed that none of them have an exit button. On Ubuntu the idea is that you just slide apps into the background.
If you want to make a C++ application with a HTML5 UI you should read the answer to this question.
Many of the features on that table are extremely new. The Media Source Extension spec, for example, is still a draft and is still being written. And it's being driven by Google and Netflix. It's going to take time for the spec to mature to a point where Mozilla want to spend any time implementing it.
Update: In Firefox 30 (should be the default in all supported versions now), the feature is there but it's disabled by default. This should change in Firefox 31 but you can enable MSE by going to about:config (stick that in the address bar) and searching for mediasource. That should leave you with the media.mediasource.enabled option. Double click to enable.
On the Youtube panel the MSE & H.264 option is still crossed out... But the other two MSE options are now green.
Best Answer
Depends on the site. Firefox has context menus for saving html5 video, but they don't work if the site uses a customized player. For example, if you visit http://www.mozilla.com/en-US/firefox/video/, you will be able to right-click and select the Save video option.
On YouTube, that won't work. In this case, click the YouTube logo in the address bar, then click More information in the mini popup, then click the Media tab in the big popup dialog, then find the video and select it, then click Save As. Change the extension to webm before saving.
Alternatively, you can copy the address and load it on some download manager like DownThemAll or
wgetvia terminal.However, on YouTube is simpler to use Video Download Helper.