Our free XML sitemap validator tool will validate your sitemap files against the official sitemaps.org specification and inform you of any issues that make the sitemap invalid.
Strict validation will run the XML files against the official spec and fail the sitemap if they do not meet the exact specification.
Non-strict mode will simply try to read the URLs from the XML sitemap files. This is the approach that Google Search Console takes.
If you need to generate an XML sitemap, try our Sitemap Generator.