Why would a solid designed web page, using valid HTML, CSS and only a minimum of JS, for extended functionality, be a cross browser, or even browser extension problem? These only come up, when you start to use JS for everything, instead of doing proper web development, IMHO. You do not have to be exceptionally smart to do this, you just have to know your trade, and must not fell to all that enerving animated wanna-be-impressive stuff some think would be great design. As, if you think of these pages done by web designers, which have no real clue how to do stuff, but instead want to show how 'great design' they can do and how great it would be if the whole page moves around all the time, letting you wait for content to load (very) lazy, instead of just delivering it, OK, but a solid developed web page?

And stuff like games come with well tested JS frameworks, where you (should) have no problems if you do not start to fiddle around with required 3rd party cookies, blocking non-referer-sending requests, and other senseless stuff.