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.
mfcss i am also facing this issue from last night and this issue is related to google page speed insights new update ( all of my client shopify stores are getting this error while checking on google page speed insights new updated tool .
Try checking from you shopify dashboard click on view report under theme and then click on view report for all pages homepage, product page and collection pages this might work.
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.
here is an update as it looks like a shopify issue with new tool but found a solution if you make a copy of theme and check by preview link in google page speed insights it shows the mobile and desktop score