Hi Polonus,
look the thing is as you know that it's not strictly legal to download videos when there's not an explicit link to do so, meaning that the author/owner prefers that his work would be streamed only. So, when it's doable it's doable, via any FF extension or else, and when it's not it's not and I won't complain about it. Downloading videos on the internet is a technical possibility, not a right

You're watching what's been put online by someone who made a video, and the sharing conditions are up to this someone in the end. If HTML5 became a further restriction I don't really care, but I doubt it. HTML5 for a start shouldn't allow all the tracking and privacy concerns there has always been with Adobe/Macromedia flash player. Once you've cleared your private data in Firefox, there won't be anything left when using HTML5. Currently you still have to take care of the macromedia folder in your user profile, so that's already a plus for HTML5... seriously, downloading will always be possible, you're getting the streaming data, nothing can prevent you from caching it and save it to disk what ever the format is. MediaPlayer can read anything, ON2...FLV...and H264...

That said embedding might be prevented, and that wouldn't be a good idea, for bloggers...or any other site btw.