Validate the HTML of a page or a snippet with the W3C validator.
The HTML validator checks a web page or the HTML you paste against the HTML standard, with the W3C Nu HTML Checker. Every error and warning is listed with its line, column and the code around it.
Valid HTML renders the same in every browser, is easier for assistive technology to read and avoids surprises such as broken layouts from an unclosed tag.
The official HTML5 validator.
Validate a live page or a snippet.
Syntax highlighting while you paste and edit.
Every message with its position and extract.
Small errors don't, as browsers and Google are tolerant, but serious ones like unclosed tags can hide content.
An unclosed or misplaced tag affects everything after it. Fix the first errors and validate again.
Errors break the HTML standard; warnings point to things that are allowed but not recommended.