:muted

The :muted CSS pseudo-class selector represents an element that is capable of making sound, such as <audio> or <video>, but is muted (forced silent).

Muted is different from :volume-locked in that the page author has control over whether a media element can be muted or un-muted. User agents may set media muted value according to use preferences (e.g., remembering the last set value across sessions, on a per-site basis, or otherwise). An element that is :volume-locked cannot be muted, un-muted, or have its volume changed via JavaScript because of an operating system or user agent preference.

:muted {
  /* ... */
}

:muted {
  outline: 5px solid red;
}

video:muted {
  outline: 5px solid blue;
}