I'm genuinely curious: why is that exciting? I don't know a huge amount about the web stack.
Your typical "CSS3 demo" on HN will work well on Opera, Firefox, Chrome, Safari. It should work on IE10, has a 50/50 chance of working on IE9, and probably won't work on IE8 and below.
Check out http://caniuse.com/ to see which features are supported on each browser.
One other thing to mention is that users might turn off JavaScript support and screenreaders will have issues with some things JavaScript apps do. It was much more of an issue 5-10 years ago than it is today though.