HTML Validator

A single unclosed <div> can push every following element inside it. Your layout may survive by luck, but selectors, event delegation, and screen reader navigation all read the resulting tree, and each of them will behave in ways that are hard to debug from the symptoms alone.

Structural ones: elements that were opened and never closed, closing tags that do not match what is currently open, stray closing tags with nothing to close, and deprecated presentational elements such as <center> and <font>.

No. The W3C service checks your document against the full HTML specification, including attribute-level rules, and it needs to send your markup to a server. This is a structural check that runs locally and catches the mistakes that actually break rendering. Use the W3C validator when you need formal conformance.

Browsers implement aggressive error recovery and will guess where a missing tag belonged. The trouble is that different browsers guess differently, and your CSS selectors and DOM scripts then operate on a tree you did not intend. Fixing the structure removes that uncertainty.

No. Elements such as <img>, <br>, <input>, and <meta> have no content and are never closed, so the validator does not expect a matching tag for them.