What does an automated accessibility audit check?
Three checks, with a combined weight of 4.5: the Lighthouse accessibility score (weight 2.5), the share of images that have alt text (1.5) and the presence of a lang attribute (0.5). Lighthouse is 2.5 of 4.5, so it is more than half the category on its own. This is the smallest check list in the rubric, which is deliberate, because automated tools can only reach a fraction of accessibility.
The alt text check is a ratio, not a pass or fail. It is one minus the number of images missing alt divided by the total number of images, so a page with 20 images and 4 missing alt scores 0.8 on that check. A page with zero images scores 1.0, because there is nothing to fail.
The lang check is presence only. An html element with a non-empty lang attribute scores 1.0 and one without scores 0.0. There is no check on whether the declared language is correct, only that one is declared.
Everything else that is normally called accessibility, colour contrast, button and link names, ARIA usage, form labels, reaches the score through the Lighthouse accessibility number rather than through its own check. That score is averaged across the mobile and desktop runs when both are available.
This is an automated audit and it does not replace manual testing. Automated tooling cannot judge whether alt text is meaningful, whether keyboard focus order makes sense, or whether a screen reader can complete a task. A high score here means no automated failures were found, not that the site is WCAG conformant. Accessibility carries a weight of 0.9 in the composite score.
What is measured, and how it is scored
3 checks make up the Accessibility score. Weights are relative within the category and sum to 4.50.
| Check | Rule | Weight |
|---|---|---|
| Lighthouse accessibility category score, averaged across the mobile and desktop runs | The 0 to 100 score divided by 100. This is where contrast, element names and ARIA problems enter the score. | 2.5 |
| Share of images that have an alt attribute | One minus images missing alt divided by total images. 4 of 20 images missing alt scores 0.8. A page with zero images scores 1.0. | 1.5 |
| Presence of a lang attribute on the html element | Presence only, no threshold. A non-empty lang value scores 1.0 and an empty one scores 0.0. The value itself is not validated. | 0.5 |
Accessibility questions
Does a perfect accessibility score mean the site is WCAG compliant?+
No. The score comes from three automated checks, the Lighthouse accessibility number, the ratio of images with alt text, and whether a lang attribute exists. Automated tooling cannot confirm that alt text is meaningful or that a screen reader user can finish a task, so a high score means no automated failures were detected, not conformance.
How is the image alt text check scored?+
It is a ratio: one minus the number of images missing alt divided by the total number of images. A page with 10 images where 2 lack alt text scores 0.8 on that check, which is worth 1.5 of the category's 4.5 total weight.
What if a page has no images at all?+
It scores 1.0 on the alt text check. There is nothing to fail, so the check gives full credit rather than being skipped, which would shift weight onto the Lighthouse score.
Where does colour contrast get scored?+
Inside the Lighthouse accessibility score, which carries weight 2.5 of the category's 4.5. There is no separate contrast check in the rubric, so contrast problems move the number only as much as Lighthouse weights them.
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