How do you test whether a website works on mobile?
Four checks, with a combined weight of 5: the mobile Lighthouse performance score (1.5), a viewport meta tag (1.5), the gap between desktop and mobile performance (1) and mobile Cumulative Layout Shift (1). The gap check gives full credit when mobile trails desktop by 10 Lighthouse points or fewer, and zero at a gap of 45 points or more.
The gap check is the one that is specific to this category. A site can score acceptably on mobile in isolation while being dramatically worse than its own desktop version, and that difference is the signal: the page was built and tuned for a large screen. Only a positive gap counts. If mobile beats desktop, the gap is treated as zero and earns full credit.
The viewport check is presence only. A non-empty viewport meta tag scores 1.0 and a missing one scores 0.0. The contents of the tag are not parsed, so this check confirms that responsive layout was attempted, not that it was done well.
Mobile CLS is scored against the same bounds as everywhere else in the rubric, full credit at 0.10 or below and zero at 0.25 or above. Here it is read specifically from the mobile run rather than falling back to desktop, because layout shift on a narrow screen is a different problem from layout shift on a wide one.
There is no separate tap target or touch ergonomics check in the rubric. Whatever the mobile Lighthouse run scores for those is already inside the 1.5 weight on the mobile performance number. Mobile carries a weight of 0.9 in the composite score.
What is measured, and how it is scored
4 checks make up the Mobile score. Weights are relative within the category and sum to 5.
| Check | Rule | Weight |
|---|---|---|
| Lighthouse performance score from the mobile run | The 0 to 100 mobile performance score divided by 100. Skipped when the mobile run produced no score. | 1.5 |
| Presence of a viewport meta tag | Presence only, no threshold. A non-empty viewport meta scores 1.0 and a missing one scores 0.0. The tag's contents are not inspected. | 1.5 |
| How far the mobile Lighthouse performance score trails the desktop score | Full credit at a gap of 10 points or fewer. Zero at 45 points or more. Straight-line partial credit in between, so a 27.5 point gap scores 0.5. A negative gap, where mobile beats desktop, counts as zero and earns full credit. Requires both runs. | 1 |
| Cumulative Layout Shift measured on the mobile run specifically | Full credit at 0.10 or below, zero at 0.25 or above, straight line in between. Unlike the performance category's CLS check, this one does not fall back to the desktop run. | 1 |
Mobile questions
How big a mobile to desktop performance gap is too big?+
A gap of 10 Lighthouse points or fewer earns full credit. Credit then falls in a straight line, hitting zero at a gap of 45 points. A desktop score of 90 against a mobile score of 50 is a 40 point gap and scores about 0.14.
Does a viewport meta tag guarantee a mobile-friendly page?+
No, and the check does not claim it does. It is a presence test worth 1.5 of the category's 5 total weight, so it confirms a responsive layout was attempted. The tag's contents are not parsed and the rendered layout at narrow widths is not inspected.
Are tap target sizes checked?+
Not as a separate check. The rubric's four mobile checks are the mobile Lighthouse performance score, viewport presence, the mobile to desktop gap, and mobile CLS. Any tap target finding reaches the score only through the Lighthouse number itself.
What happens if only the desktop Lighthouse run succeeds?+
The mobile performance check, the gap check and the mobile CLS check are all skipped, because each needs a mobile run. The category would then be scored on viewport presence alone, or marked unmeasured if that could not be observed either.
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