List of URLs will show all your posts, pages, custom post types and taxonomy archive pages. Select the URLs you want to validate and click “validate”. You can also use the search function to filter URLs. If you have hundreds of pages – don’t worry. Nothing bad will happen, it’ll just take longer for the validation to finish. It takes about 4 seconds per URL to validate.

Once validation is done you’ll immediately see the number of errors and warnings separately for HTML and CSS. additionally there’s a “details” button available. Clicking it will give you a detailed report. Click the “zoom” icon next to each error to get detailed info.

Validation results are cached so once you do the validation they’ll be available without running it again. If you get an error response for some pages it means that the W3C validator can’t access them or the W3C service is temporary not available.