[BUG] WebKit doesn't play webm files on MacOS

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Triaging summary from @aslushnikov: WebKit doesn’t play webm files on MacOS.

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Context:

• Playwright Version: 1.27.1
• Operating System: macOS 12.6 on M1
• Node.js version: 16.17
• Browser: WebKit

## System:
 - OS: macOS 12.6
 - Memory: 214.27 MB / 32.00 GB
## Binaries:
 - Node: 16.17.0 - ~/.nvm/versions/node/v16.17.0/bin/node
 - Yarn: 1.22.19 - ~/.yarn/bin/yarn
 - npm: 8.15.0 - ~/.nvm/versions/node/v16.17.0/bin/npm
## Languages:
 - Bash: 3.2.57 - /bin/bash

Describe the bug

The WebKit version fetched by Playwright has an unexpected user agent (reported with navigator.userAgent):

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_2) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.0.4 Safari/605.1.15

In particular, I would expect Version/16.0, which I correctly get with my default Safari. With a manually installed Safari Technology Preview, I get Version/16.4.

I’ve tried cleaning caches and fetching it again:

$ rm -rf ~/Library/Caches/ms-playwright/webkit-*
$ npx playwright install webkit
Downloading Webkit 16.0 (playwright build v1724) - 53.9 Mb [====================] 100% 0.0s
Webkit 16.0 (playwright build v1724) downloaded to ~/Library/Caches/ms-playwright/webkit-1724

I suspect that the current build is not actually WebKit 16, but WebKit 13. I cannot be sure although WebM video playback support (don’t play in this build, plays in official Safari and STP) seems to confirm it.

I’ve also tried forcing build 1728 (latest unreleased) to no avail.

[aslushnikov]

@warrenseine the WebM should be supported on MacOS; I indeed see it doesn’t play. We’ll take a look.

[warrenseine]

Yes, I used the Web Inspector.

If it is indeed version 16, can you confirm that WebM is not supported? Any reason why we see these differences in feature support?