Impact and number of elements
Each rule has a weight: critical 10, serious 6, moderate 3, minor 1. The penalty is weight × (1 + log₂ of affected elements), capped at 3 × weight. The score starts at 100 and subtracts penalties, never below zero.
Caps for critical issues
For each page and viewport, one rule with a critical issue caps the score at 79; three or more distinct critical rules cap it at 59. The page uses the lower desktop/mobile score. The overall score is the weighted average of scanned pages, with the home page counted twice. The 79 and 59 caps are not applied again to the overall score.
What automation cannot measure
The usefulness of an image description, real usability, accurate captions and many other needs require human review. Custom heuristic checks can need confirmation.