:seeking

The :seeking CSS pseudo-class selector represents an element that is playable, such as <audio> or <video>, when the playable element is seeking a playback position in the media resource. A resource is considered to be seeking if the user has requested playback of a specific position in the media resource, but the media element has not yet reached that position.

Seeking is different from :buffering in that the media element is not currently loading data, but is instead skipping to a new position in the media resource. For more information, see the Media buffering, seeking, and time ranges guide.

:seeking {
  /* ... */
}

:seeking {
  outline: 5px solid red;
}

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