Is it :modal?

This handy CSS pseudo-selector gives you a way to select elements that are modal.

Jhey Tompkins

This handy pseudo-selector gives you a way to select elements that are "modal", and to avoid managing classes in JavaScript by providing a way to detect modal elements.

Two types of elements are currently classed as :modal:

• Dialog elements using the showModal method.
• Elements that are in full-screen mode.

How can you use it? This example sets the scale of all <dialog> elements that are :modal.

dialog:modal {
 scale: 2;
}

Consider this demo where you can show a <dialog> in either "modal" or "non-modal" styles.

When you show the "modal" version, it uses the ::backdrop provided by the top layer.

dialog::backdrop {
 background: hsl(0 0% 10% / 0.5);
}

But, for the non-modal version which doesn't have a ::backdrop, a fake one gets created with the ::before pseudo-element. It's lighter and doesn't blur the content behind it. You could combine with :not to detect a non-modal <dialog>.

dialog[open]:not(:modal)::before {
  content: "";
  position: fixed;
  height: 100vh;
  width: 100vw;
  top: 50%;
  left: 50%;
  background: hsl(0 0% 10% / 0.25);
  transform: translate3d(-50%, -50%, -1px);
}

This will also work for elements that are in full-screen mode too. Consider this heading element made of spans.

<header>
  <h1>
    <span style="--index: 0;">:</span>
    <span style="--index: 1;">m</span>
    <span style="--index: 2;">o</span>
    <span style="--index: 3;">d</span>
    <span style="--index: 4;">a</span>
    <span style="--index: 5;">l</span>
 </h1>
</header>

When the element is in full-screen mode, the spans will animate.

h1 span {
 animation: jump calc(var(--speed, 0) * 1s) calc(var(--index, 0) * 0.1s) infinite ease;
}
header:modal span {
  --speed: 0.75;
}
@keyframes jump {
  50% {
    transform: translateY(-50%);
  }
}