<responsive-embed>

Keeps embeds at a fixed aspect ratio. Source and docs.

Inferred from the embed code

No ratio attribute: the YouTube snippet's width="560" height="315" gives 16:9.

<responsive-embed>
  <iframe width="560" height="315" src="https://www.youtube.com/embed/fCLMI5TCcqg" allowfullscreen></iframe>
</responsive-embed>

Explicit and reactive ratio

An invalid value (try 16x9) logs a warning in the console and falls back to 16:9.

Any content

A <video> with its own dimensions: