A new at-rule in CSS, @supports, allows you to easily detect if a certain CSS feature is available or not in the visitor’s browser. This allows you to make use of these features only for supported browsers or define specific rules only for browsers without support, and have other browsers completely ignore them. @supports is a pure CSS way to replace a tool like Modernizr. @supports allows you to test for both the support of a feature and the non-support of a feature:

@supports (column-count: 3) {
  div { column-count: 3; }
}

Non-support

Here’s how you would test and apply specific rules when a feature is not supported:

@supports not (display: flex) {
  .sidebar { float: left; }
}

Chain multiple CSS features using and / or operators:

@supports (display: flex)
or (display: -webkit-box)
or (display: -webkit-flex)
or (display: -ms-flexbox) {
 .menu {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
 }
}

One big caveat

There’s one major caveat to @supports right now: @supports itself is not universally supported. It’s not supported in Internet Explorer at all, not even version 11, and is has only been supported in Safari for iOS since v9.2. See the @supports Can I Use page for more details.

Because of that, it might be more practical to continue using Modernizr for some time, until most IE users have switched over to Edge.

@supports also has an handy JavaScript API that can be used to detect for features in JS.