What a good sitemap looks like
An XML sitemap lists the pages you want in search results, so search engines can find them all. Each sitemap holds at most 50,000 addresses and 50 MB uncompressed; bigger sites split it into several files listed in a sitemap index.
Every address in it should load with status 200, not redirect, not be marked noindex, and name itself as the canonical page. Listing pages that fail those checks wastes crawling and weakens how much Google trusts your sitemap.
What this checker does
It finds your sitemaps from robots.txt and the usual addresses (or uses the one you enter), follows sitemap indexes, and checks the XML, namespace, size, entry count, lastmod dates, duplicates, other-domain and http:// addresses. Then it picks 20 addresses spread across the sitemap and checks each one's status, redirects, noindex tags, canonical tag and robots.txt rules.
Google ignores the priority and changefreq tags, so they're optional. Accurate lastmod dates are worth keeping.