Validator
The same engine the command line runs, and the same check ids. Every finding names the field, states the rule, and tells you what to change.
Check a live URL
Point it at your manifest or a feed. The validator downloads what is published and reports what it found, including the transport behaviour a file on disk cannot show.
Check a document you have not published yet
Paste a feed or manifest. Useful before anything is deployed, and for a document you would rather not send anywhere.
This runs entirely in your browser. The document is not uploaded, not logged, and never leaves this page.