We are trying to use https://pagespeed.web.dev/ to evaluate the speed of our site.
The tool has worked on our site URLs for months, but for the past few days we have been getting below error when trying to use it:
Lighthouse returned an error: NOT_HTML. The specified page is not HTML (displayed as MIME type text / plain).
Our site is a fairly standard Shopify page, see e.g. this URL for replication of the issue.
We have now learned from the github page of Lighthouse that this is related to a lack of support for Early Hints (used by Shopify) in the Lighthouse 9.0.0 release.
As suggested, there are short-term workarounds, e.g. using Lighthouse via the Chrome Developer Tools works for now. Shopify technical support has also been notified of this problem.