Only 2.6% of the most visited websites have fully valid HTML

A study of the 2,655 most visited websites found that 90.5% contain HTML errors and only 2.6% have completely valid markup, revealing that valid HTML is rare despite browsers' ability to recover from errors. The most common failures are structural issues caused by frameworks composing elements without knowing their context, and accessibility problems like missing heading levels and unlabeled inputs appear on over a third of sites. CSS errors are also widespread at 65.1% of sites, though typically limited to one or two broken declarations that browsers silently ignore.