Answer 1

You can't do it with just HTML. You'll either need a dynamic web server running PHP or Python or Ruby or ... script(s), or some really clever JavaScript (Google Firebase would be pretty helpful). As to the code itself, I'm not that good/experienced enough to be able write it myself.
If you do wish to try it out yourself (extracting videos from sites like YouTube) my advice would be to look at how they host their videos, and how other video downloading websites do it. I find Chrome Developer Tools pretty helpful for doing that.