Only 2.6% of the Web's Top Sites Have Valid HTML

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

A study of the top 5,000 websites found that 90.5% have at least one HTML error, with a median of 12 errors per site. Only 2.6% are completely clean. The most common issues are structural nesting problems and accessibility failures, while CSS errors are less frequent but still affect 65.1% of sites. The research highlights that browsers' error recovery masks these issues, allowing them to persist.