[LukeShu]

So, I know that was a joke, but now I'm trying to figure out why it's a bad idea.

The browser can generate some kind of map for which region of the screen is what font. If you don't have to guess the font, OCR should be easy and reliable. That takes care of the hidden text issue. But second, it means one would be able to copy/paste text that is in an image (because some web designers hate you).

[jiggy2011]

But you don't select text by selecting an area technically, you select a sequence of characters.

You could do a per character visibility test at the time of copying, but sometimes you want to copy test that is not currently visible on your screen.

For example doing Ctrl+A in a document.