In practice
Offer it as the first source with an MP4 fallback, and never autoplay a background video that is not muted and compressed.
A short WebM loop is usually a better choice than an animated GIF, which can be an order of magnitude larger.
Why it matters
Video is the heaviest asset most pages carry, so the format choice dominates its cost.
Smaller media keeps hero sections usable on mobile connections instead of stalling the first paint.
Related terms