The @supports rule's ability to check for selector support is surprisingly robust! While often used to test for property: value pair compatibility, the selector() function extends its capabilities to evaluate selector support. This is done by simply placing the selector within the parentheses:
@supports selector(:nth-child(1 of .foo)) {
/* Styles applied if the selector is supported */
}
The :nth-child(n of .foo) selector, a "selector list argument" compatible with :nth-child family selectors, serves as a good example. Currently, only Safari supports this specific selector.
Consider a scenario where you need to style a list with separators and zebra striping. Ideally, you'd use a selector like this to achieve the zebra striping while ignoring separators:
li:nth-child(odd of .list-item) {
background: lightgoldenrodyellow;
}
However, due to limited browser support, you can use @supports to conditionally apply this style:
@supports selector(:nth-child(1 of .foo)) {
li {
padding: 0.25em;
}
li:nth-child(odd of .list-item) {
background: lightgoldenrodyellow;
}
li.separator {
list-style: none;
margin: 0.25em 0;
}
}
@supports not selector(:nth-child(1 of .foo)) {
li.separator {
height: 1px;
list-style: none;
border-top: 1px dashed purple;
margin: 0.25em 0;
}
}
This provides a fallback style for browsers lacking support for the advanced selector. An improved syntax, potentially using @when and @else, could simplify this:
/* Hypothetical future syntax */
@when supports(selector(:nth-child(1 of .foo))) {
/* Styles for supporting browsers */
} @else {
/* Fallback styles */
}
A JavaScript API also exists for testing selector support:
CSS.supports("selector(:nth-child(1 of .foo))")
This method returns true in Safari and false in Chrome (at the time of writing).
While the @supports rule is powerful, the number of CSS selectors with inconsistent cross-browser support, and the number of use cases requiring @supports for those selectors, is relatively small. Many previously problematic selectors, such as ::marker and case-insensitive attribute selectors, now enjoy broad support. Selectors like :fullscreen or :-webkit-full-screen might offer interesting and useful applications due to their unique lack of support in iOS Safari.
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