HTML Validator

What gets checked

Your markup is checked against the current HTML standard: tag nesting, required attributes, deprecated practices, duplicate IDs, and errors in attribute values and in inline styles.

Why errors are grouped

A page with the same error on fifty images produces fifty identical lines, and everything else gets buried under that wall. Here, identical errors are collapsed into one line with a repeat count and line numbers, and the matching piece of markup is shown alongside with the problem spot highlighted.

Errors explained in plain language

Validators write dry, terse messages. For common errors this tool adds a plain-language name and an explanation of what it means in practice: what the visitor sees, what the browser does, and how indexing is affected. The original wording stays next to it, which is handy for searching and cross-checking with other validators.

Three ways to check

By URL: we fetch the page and parse the code the server returns to a robot. By pasted code: when the fix is not published yet. By a list of up to 100 URLs: it shows that the same error lives in a shared template and gets fixed once, not on every page.

What counts as an error and what does not

An error breaks the standard, and browsers repair it their own way: the layout may fall apart in one and hold together in another. A warning flags a practice the standard allows but considers questionable. A notice can usually stay as is: these are traces of older HTML versions, like a slash at the end of a self-closing tag.