About the Sitemap Validator
This fetches your sitemap, parses the XML, checks the structure against the sitemaps.org schema and then samples the URLs inside to see whether they actually resolve.
The failures worth acting on are the ones that make the file useless rather than merely untidy. URLs that 404 or redirect tell Google your sitemap is out of date, and a file full of stale entries gets crawled less often. URLs on a different hostname than the sitemap are ignored outright. So are URLs that your own robots.txt blocks — you are simultaneously asking for a page to be crawled and refusing to let it be.
Size limits apply too: 50,000 URLs and 50 MB uncompressed per file. Past that you need a sitemap index pointing at several files, which is also the cleaner structure for any site large enough to care, because it lets you see in Search Console which section of the site has coverage problems.
Common questions
Why does a URL in my sitemap fail even though it loads fine?
The validator checks the exact URL listed. A redirect, a different hostname (www vs non-www), or a trailing slash mismatch between the sitemap entry and the live page all count as a mismatch worth fixing.
What's the maximum size for one sitemap file?
50,000 URLs or 50 MB uncompressed, whichever comes first. Larger sites need a sitemap index file pointing at several smaller sitemaps.
Should blocked URLs be in the sitemap?
No. Listing a URL that your own robots.txt disallows sends a contradictory signal — you're asking for it to be crawled and refusing access at the same time.