As part of my daily job as a web developer, I am constantly plagued with the different implementations of CSS layout definitions in different web browsers. The most useful website I’ve found to use over the past year is “Can I Use” by Alexis Deveria, which contains a maintained and comprehensive list of which technology is supported in which browsers.
If you’re using a web browser in which you can type a search term directly into the browser toolbar, such as Google Chrome, you can simply enter caniuse followed by the feature. The first result is then (almost) always the information page at caniuse.com.
For example, finding out whether you can use CSS gradients in all browsers is easy: the search term caniuse linear-gradient takes you straight to the CSS gradients page, which tells us that the technology is supported according to the W3C standards in all current browsers except Opera Mini.