How to use @supports in CSS to specify CSS that targets browsers capable of implementing a certain feature
So when do you want to use @supports? A Feature Query is a tool for bundling together CSS declarations so that they’ll run as a group under certain conditions. Use a Feature Query when you want to apply a mix of old and new CSS, but only when the new CSS is supported.
Let’s look at an example using the Initial Letter property. This new property initial-letter tells the browser to make the element in question bigger — like for a drop cap. Here, the first letter of the first word in a paragraph is being told to be the size of four lines of text. Fabulous. Oh, but I would also like to make that letter bold, and put a bit of margin on its right side, and hey, let’s make it a nice orange color. Cool.
We don’t want to change the color of the letter, or add a margin, or make it bold unless it’s also going to be made bigger by the Initial Letter property. We need a way to test and see whether or not the browser understands initial-letter, and only apply the change to color, weight, and margin if it does. Enter the Feature Query.
Read more from the source: hacks.mozilla.org