Some mistakes only show in the finished HTML: two elements with the same id, an image without alt text, a form field nothing labels, a link to a page that does not exist. With $padCheckOutput = TRUE in _config/config.php, PAD reads every local HTML response back once it has rendered and names them.

PHP fragments/output_checks_1.php	HTML fragments/output_checks_1.pad	Result
`<?php $findings = padOutputCheck ( '<img src="logo.png"><input name="email"><a href="?no_such_page">More</a>' ); ?>`	`<ul> {findings}<li>{$findings}</li>{/findings} </ul>`	<img src="logo.png"> has no alt text <input name="email"> has no label <a href="?no_such_page"> is a broken link: there is no page 'no_such_page'

The findings appear in a panel at the end of the page, before </body>, and their number in a PAD-Output-Check header. A page with nothing to report is left exactly as it was.
The link check is exact: a ?page link is broken when the router finds no page for it - in this application, or in the one a /<app>/?page link names.
Only a local request is checked: the command line, or this machine asking itself. A remote visitor never sees the panel. One request can ask for it with ?page&padCheckOutput.
develop/?links runs the link check over the templates of every application at once, for every link written literally in them.