CSS Usage gatherer causes console errors

#### Provide the steps to reproduce
1. Run LH on https://pack11ty.dev/

#### What is the current behavior? Errors in console fails with a message about a 503

#### What is the expected behavior? Errors in console passes. If you run with `--skip-audits=font-display --skip-audits=unused-css-rules --skip-audits=render-blocking-resources --skip-audits=unminified-css` the category gets a 100

#### Environment Information
* Affected Channels: *
* Lighthouse version: 6.1.1

**Related issues**
https://twitter.com/nhoizey/status/1284734497246851072