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

[huntharo]

Describe the bug

Avoid enormous network payloads is getting flagged for a page with two <source> tags within the <video> 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 <video> <source> 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

[adamraine]

Yeah this is happening on web.dev and PSI but never DevTools or CLI.

I tried launching Chrome with --headless and --headless=chrome on the CLI but it still wouldn't repro.

[ErwinAI]

Having the same problem here!