[kemayo]

That "full screen" button in the top-right.

Or, at least, that's the only thing I see there that you can't do with HTML/JS.

[personalcompute]

You can actually, F11

[kemayo]

Well, yeah, but you can't have a handy button for it on the page.

[bricestacey]

Not in Safari. It doesn't have fullscreen.