Document doesn't use legible font sizes
Many search engines rank pages based on how mobile-friendly they are. Font sizes smaller than 12 px are often difficult to read on mobile devices and may require users to zoom in to display text at a comfortable reading size.
How the Lighthouse font size audit fails
Lighthouse flags pages with font sizes that are too small to read easily on mobile:
Lighthouse flags pages on which 60% or more of the text has a font size smaller than 12 px. When a page fails the audit, Lighthouse lists the results in a table with four columns:
| Source | The source location of the CSS ruleset that is causing the illegible text. |
| Selector | The selector of the ruleset. |
| % of Page Text | The percentage of text on the page that is affected by the ruleset. |
| Font Size | The computed size of the text. |
Each SEO audit is weighted equally in the Lighthouse SEO Score, except for the manual Structured data is valid audit. Learn more in the Lighthouse Scoring Guide.
How to fix illegible fonts
Check font sizes in your CSS. Aim to have a font size of at least 12 px on at least 60% of the text on your page.
How to fix a missing viewport config
If Lighthouse reports Text is illegible because of a missing viewport config, add a <meta name="viewport" content="width=device-width, initial-scale=1"> tag to the <head> of your document.
See the Does not have a <meta name="viewport"> tag with width or initial-scale post for more information.
Resources
Source code for Document does not use legible font sizes audit