Don’t comma-separate :focus-within if you need deep browser support

I really like :focus-within. It's a super useful selector that allows you to essentially select a parent element when any of its children are in focus.

Say you wanted to reveal some extra stuff when a <div> is hovered...

div:hover {
  .extra-stuff {
     /* reveal it */
  }
}

That's not particularly keyboard-friendly. But if something in .extra-stuff is tab-able anyway (meaning it can be focused), that means you could write it like this to make it a bit more accessible:

div:hover,
div:focus-within {
  .extra-stuff {
     /* reveal it */
  }
}

That's nice, but it causes a tricky problem.

Browsers ignore entire selectors if it doesn't understand any part of them. So, if you're dealing with a browser that doesn't support :focus-within then it would ignore the CSS example above, meaning you've also lost the :hover state.

Instead:

div:hover {
  .extra-stuff {
     /* reveal it */
  }
}
div:focus-within {
  .extra-stuff {
     /* reveal it */
  }
}

That is safer. But it's repetitive. If you have a preprocessor like Sass...

@mixin reveal {
  .extra-stuff {
     transform: translateY(0);
  }
}
div:hover {
  @include reveal;
}
div:focus-within {
  @include reveal;
}

See the Pen
Mixing for :focus-within without comma-separating by Chris Coyier (@chriscoyier)
on CodePen.

I'd say it's a pretty good use-case for having native CSS mixins.

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