HI welcome to the forum.

I don't use HMUA myself, butI will take a guess... It would seem that it is masking the device information that is usually given to a website when the handshake occurs. This would mean that Youtube will display the full webpage and all of the options that you would see in a full desktop web browser instead of adapting (or redirecting) the page for specific device browsers.

As for the videos themselves, well they won't be displayed in 1080p on the n900 as we know we have 800x480 display and MicroB will be doing its job of scaling web pages to fit the screen, so only the embeded player will be running at full HD.

That said, a good 1080p video can still look pin sharp I may have to have a play with Hide My User Agent.

MIN