Check documents before they leave the editor. Check the active document, review the results and return to the source to correct markup issues before publishing.
Run Tools ▸ Validate ▸ Current Document (W3C) (Shift+Alt+W) to send the active HTML source to the online validator. This check needs an internet connection. Live Document (W3C) is shown in the menu but is not available yet: validating the live document needs the rendered page's HTML from the preview.
Read the reported errors, warnings and other messages while the document is open.
Select a result with a location to reveal the relevant line in the editor. Correct the markup and run the check again.
Use Tools ▸ Validate ▸ As XML (Shift+Alt+X) for a local well-formedness check. This checks XML syntax rather than HTML conformance.