Always display YouTube videos in 16:9 aspect ratio

[AprilSylph]

Outbox-for-Tumblr/src/outbox.css

Lines 441 to 443 in a2e9e4e

	[data-block="video"] iframe[id="youtube_iframe"] {
	height: calc(540px * (9/16));
	}

This CSS assumes the video width is 540px, which is usually correct. However, post width can shrink according to the viewport. The universally correct value would be:

height: calc(min(100vw, 540px) * (9 / 16));