Usually JavaScript would be used to create the flyout menu effect like you see in the Dabblet. But since it's done in CSS3 any browsers which support those standard properties can work - no need for any JS code. It's just a neat workaround and very impressive IMO.
All mainstream browsers support JS and CSS, the question is how recent and what specific features. "CSS3" really means a whole ton of features and support varies.
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.
Do any browsers with any real marketshare (even a tenth of a percent) not support JavaScript? I can't think of any examples although I've never had any reason to dive too deep into browser compatibility specifics for my projects.
