What happens when we use a feature that is unsupported by another browser?

Answer 1

Every browser is different, but most browsers will just ignore the tag.

Answer 2

most features are supported. there are vendor prefixes for features that aren't fully supported. -webkit-transition: all 0.75s ease; you will sometimes have several prefixes along with the standard style. there is a cool site I use called autoprefixer that adds these vendor prefixes to your css if you need them. also check out http://caniuse.com to see what is supported

Answer 3

I agree with Gabe rust, if unsupported the feature will be ignored by the browser