Context
Pages moved between hosts, previews and directory URLs, and relative links broke when a page moved one level. Some links ended in .html, so one page answered at two URLs.
Decision
Internal links are root-absolute directory URLs (/man/, /style.css), never relative and never ending in .html. Generators and hand-written pages follow the same rule, and the gate enforces it.
Consequences
Every URL has one form and survives a move. A link written inside a JavaScript template string reads as relative to the check, so scripts build links with a.href in code. See Link check flags a template string.