You may probably remember the earlier version of the Page Diagnostics tool for SharePoint Online.
it was about how you can use the Page Diagnostic tool to analyze your classic publishing pages and pages on classic team sites, against a subset of recommended practices in SharePoint Online.
There is now a fresh release of the
Page Diagnostics tool for SharePoint Online Version 2
New is that it now supports also “modern pages”!
See also the post from my good friend, Stefan Gossner for details 
Links:
SharePoint mysteries and more... 