The State of Browser Support

Another consideration that will usually go into whether or not you use a piece of CSS3 is how well-supported it is by major browsers, or the browsers of your particular users. While there are times when you may add more experimental and poorly supported CSS, perhaps as a little Easter egg for a particular browser, usually it’s not practical to spend time adding CSS that will be seen by only a tiny sliver of your audience.

But in order to know which pieces of CSS3 are going to be seen by a good chunk of your audience, you need to know what browsers are currently in wide use.