Well there are three issues you mention:
- each browser renders text slightly differently, because they all have different rendering engines
- tabs are paused when inactive by design (can you explain more about why this is a problem?)
- an IE bug with a warning
I'd like to know more about the IE issue because we may be able to work around it. If you let us know about issues you encounter, we can either help you solve them or come up with workarounds. I'm also sure there'd be as many, if not more, "annoyances" working in Flash - HTML5 is pretty good these days.