Measure - Multiple `<video>` `<source>` tags all reported as downloaded

**Describe the bug**

`Avoid enormous network payloads` is getting flagged for a page with two `` tags within the `` tag and both formats are getting reported as having been downloaded and contributing to the total page weight.

This does not happen in Lighthouse in DevTools. I don't *think* this should be counting both on `web.dev` either?

**To Reproduce**

Steps to reproduce the behavior:

1. Go to https://web.dev/measure/?url=https%3A%2F%2Fwww.shutterstock.com%2Fvideo%2Fclip-1079039945-solar-power-plant-windmills-aerial-view-renewable
2. Click on Run Audit
3. Scroll down to Performance -> Diagnostics -> Avoid enormous network payloads
4. Expand section
5. See that both the `webm` and `mp4` are listed as contributing to total page weight

**Expected behavior**

This isn't how `` `` works? It doesn't download both videos, it downloads one in the preferred format...

When run in Chrome DevTools Lighthouse is not showing both videos downloading, it is expected that `web.dev` would match that.

**Screenshots**