HTML Validation Issues — Reference

What each HTML Validator issue means, why it matters, and how to fix it — with links to the WHATWG HTML spec and MDN.

The HTML Validator checks pasted HTML or a live URL against a set of structural, accessibility, and SEO best-practice rules. Each issue it reports links here for a fuller explanation — what triggers it, why it matters, and how to fix it.

Unclosed & mismatched tagsTags without a matching close, or closed out of order Missing doctypeNo <!DOCTYPE html> at the top of the document Missing lang attribute<html> has no lang attribute Charset declarationMissing, or declared too late in <head> Missing <title>No page title Missing meta descriptionNo meta name="description" tag Viewport meta tagMissing, or missing width=device-width Canonical URLRelative, or pointing somewhere unexpected Duplicate id attributeThe same id used more than once Deprecated elements<center>, <font>, <marquee> Image alt textMissing or empty alt attributes Form control labelsInputs without an accessible label Heading structureMissing, duplicate, or skipped heading levels target="_blank" without rel="noopener"New-tab links missing a safe rel value