GoKawiilValid HTML is the exception, not the rule. Nine sites in ten among the most visited on the web violate the standard on their home page, and 2.6 % come back completely clean. Browsers are so good at error recovery that nothing forces anyone to fix this, which is exactly why it accumulates.
The distribution is brutally skewed. The median site has 12 errors, the mean 37.8 , and the worst 2,209 . Most sites have a handful of mistakes repeated a few times; a minority have one template bug repeated across every component on the page. If you are auditing a site, the count matters far less than the number of distinct rules broken. That is why our scores are computed per rule, not per occurrence.
The top failures are structural, not typos. The single most common finding is <X> element is not permitted as content under <X> ( 59.5 % of sites) . Nobody writes that by hand. It is what happens when components are composed by a framework that does not know which element it is being dropped into. It is a nesting problem created at build time, not in an editor.
Accessibility failures are near-universal. The findings that the Accessibility Checker keeps ( heading levels, unlabelled inputs, links with no text ) each appear on more than a third of sites. With the European Accessibility Act applicable since June 2025, that is a compliance surface, not a style preference.