Puffin's HTML Formatter & Validator is a free browser tool for cleaning up and checking markup. Paste raw HTML, drop an .html file, browse from disk, load a sample, or point at a public URL — then Process to pretty-print and validate.

The validator follows the parsing rules of the HTML Living Standard. It knows which end tags are optional, which elements are void, and how SVG and MathML differ, and it reports unclosed, stray, and mismatched tags with their line and column. Warnings cover missing alt text, missing lang and title, legacy doctypes, and obsolete elements.

With Fix HTML enabled, the tool repairs structural mistakes the same way a browser would recover from them, then formats the result. Text only wraps where it already had whitespace, and pre, textarea, and script content is kept intact. Nothing is uploaded to a server — processing runs in your browser (URL fetches are the exception, and may fail when the remote host blocks cross-origin requests).