lucy24 - 10:31 pm on Nov 15, 2012 (gmt 0)
When you are making something to go online you must must must always always always check it in many different browsers. You can also check in information sources like caniuse dot com, but there's no substitute for your own eyeballs. "Not supported" may mean it's quietly ignored-- or it may mean visual disaster.
And then you can decide what proportion of your users are expendable.
css, unlike html, doesn't identify itself. Officially both are still Under Construction-- but my most recent visits to the CSS validator quietly assumed CSS3.