How do I access and view cache HTML5 videos located in Safari or Chrome cache?
If you just want to download a video, you can use youtube-dl.

Downloading a YouTube video with Safari:


Enable using the HTML5 player from https://www.youtube.com/html5.
Enable the develop menu from Preferences > Advanced > Show Develop menu in menu bar.
Select Develop > Show Page Resources.
Reload the page.
Double-click the videoplayback resource.
Focus the location bar and press option-return.