Search-engine boundaries
Local markup checks cannot know what a crawler fetched, rendered, indexed or selected as canonical. Search engines may rewrite title links/snippets, choose different canonicals, ignore invalid or ineligible structured data, and apply live HTTP/robots conditions that are not present in pasted text. Treat these tools as pre-publish QA, then verify live behavior with the relevant search-engine tools.
Preview is a drafting aid, not a truncation oracle
Pixel width and copy length help edit snippets, but Google may rewrite title links and snippets by query/device.