Google has also updated the title links help document with a new “no clear main headline” line.
Google has added a new help document named valid page metadata that talks about how Google can process invalid or inconsistent HTML with the issues around invalid markup. Google also updated its help document on title links with a new section on “no clear main headline” in the troubleshooting section.
Valid page metadata help document
The new “use valid page metadata” help document can be found here, and it reads “using valid page metadata ensures that Google can process the HTML markup of your pages.” “Google tries to understand HTML even when it is invalid or inconsistent with the HTML standard, but errors in the markup can cause problems with your website in Google Search,” the document added. For example, if you use an invalid element in the head, Google ignores any elements that appear after the invalid element.;
This is more of an issue for schema or structured data but can potentially also impact other areas in which Google may not understand an element in your HTML.