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Is “is” Useful?

God I’m funny.

Anytime we have fairly repetitive selectors that have a common parent, it’s probably a place we can use the :is() pseudo-selector.

Holger Bartel demonstrates like this:

section section h1, section article h1, section aside h1, section nav h1,
article section h1, article article h1, article aside h1, article nav h1,
aside section h1, aside article h1, aside aside h1, aside nav h1,
nav section h1, nav article h1, nav aside h1, nav nav h1 {
  font-size: 20px;
}

Becomes:

:is(section, article, aside, nav)
:is(section, article, aside, nav) h1 {
  font-size: 20px;
}

Adam Argyle demonstrated like this:

MDN has an extra dramatic one:

ol ol ul,     ol ul ul,     ol menu ul,     ol dir ul,
ol ol menu,   ol ul menu,   ol menu menu,   ol dir menu,
ol ol dir,    ol ul dir,    ol menu dir,    ol dir dir,
ul ol ul,     ul ul ul,     ul menu ul,     ul dir ul,
ul ol menu,   ul ul menu,   ul menu menu,   ul dir menu,
ul ol dir,    ul ul dir,    ul menu dir,    ul dir dir,
menu ol ul,   menu ul ul,   menu menu ul,   menu dir ul,
menu ol menu, menu ul menu, menu menu menu, menu dir menu,
menu ol dir,  menu ul dir,  menu menu dir,  menu dir dir,
dir ol ul,    dir ul ul,    dir menu ul,    dir dir ul,
dir ol menu,  dir ul menu,  dir menu menu,  dir dir menu,
dir ol dir,   dir ul dir,   dir menu dir,   dir dir dir {
  list-style-type: square;
}
:is(ol, ul, menu, dir) :is(ol, ul, menu, dir) ul,
:is(ol, ul, menu, dir) :is(ol, ul, menu, dir) menu,
:is(ol, ul, menu, dir) :is(ol, ul, menu, dir) dir {
  list-style-type: square;
}

It’s less code and easier to reason.

Kezz Bracey notes that pairing it with :not() can be nice as well:

:not(article, section, aside) :is(h1, h2, h3, h4, h5, h6) {
  font-weight: 400;
}

Browser support is just starting to get there and polyfilling is hard, so we aren’t at day-to-day no-brainer use levels quite yet. I’d bet it’s not too far away.