We took the Tranco top 5,000 from the Tranco list , a research ranking that averages several popularity lists to resist manipulation, and requested the home page of each domain over HTTPS in August 2026.

Each page went through the same public endpoints this site runs for its users: HTML through html-validate on its standard preset, CSS through csstree-validator with lightningcss adjudicating modern syntax. Driving the real API rather than the engines means every false-positive filter we ship applied here too, so these counts are what a user of the tool actually sees.

Severity is not a style choice. We count as an error only what violates the written HTML standard. Opinions that are still valid HTML, such as an inline style or a missing type on a button, are counted separately as best practiceand are labelled as such in the tables above. Many published “error” statistics conflate the two, which roughly triples the headline.

Messages are grouped into families by masking their variable parts, so Unknown property `colr` and Unknown property `bakground` count as one problem. The exact wording shown in each row is the most frequent variant.

2,345 of the 5,000 domains produced no measurement and are excluded from every percentage on this page. A large share of the top of the list is infrastructure: CDN and DNS hostnames that were never meant to serve a page. The reasons:

• 1,479Could not reach the URL. Check that it is valid and accessib
• 433Failed to fetch: … Forbidden
• 248Response too large (max NKB)
• 81Failed to fetch: … Not Found
• 21No HTML content provided

And we ran it on ourselves. Every page of this site, all 102 of them including the one you are reading, validates at zero errors and zero warnings against the engine above. Writing this study is what made us check; it found four problems we then fixed. Publishing a number you have not applied to your own site is not research, it is marketing.