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HTML is the markup language for web pages: nested elements written as tags, with attributes and text. Build tools and CMSs often emit it on one line. Formatting adds line breaks and indentation so you can read it; the page stays the same.

Browsers never reject HTML. When a tag is left open or closed in the wrong place, they quietly guess what you meant, and the guess is not always what you intended. This validator points out where that guessing happens so you can fix the source.

Void elements such as br, img, input, and meta never have content or a closing tag. Inside svg and math, the XML-style <path /> syntax is allowed, but on regular HTML elements the slash is ignored.