Discoverability · 30 points
Favicon
One check, stated three ways: what we measure, what would make it wrong, and how to confirm it yourself.
What this measures
Whether a rel=icon link exists, whether the file answers HTTP 200, and its declared content type.
Where this check can be wrong
Some platforms ignore a missing favicon in a share card while showing one from the domain's metadata service, so the preview you saw may be friendlier than this check implies.
Verify it without taking our word for it
Open the URL in a new tab, or curl -I the href from your HTML head and check the status and content-type.