// Free HTML Dev Tool
A free html structure checker that works as a div close checker and html tag checker — catches unclosed tags, invalid nesting, missing attributes, and accessibility issues. Instant score, no login.
HTML structure validation ensures your markup follows the rules that browsers, search engines, and assistive technologies rely on. Invalid HTML can render inconsistently, break CSS, and prevent screen readers from interpreting content correctly.
Unclosed and mismatched tags
Every opening tag (div, p, ul, section) needs a matching closing tag. Unclosed tags cause browsers to auto-correct in unpredictable ways — a missing /div can collapse an entire layout. Void elements (img, br, input, meta) do not need closing tags. Self-closing syntax (/>) is optional in HTML5.
HTML div checker — why unclosed divs break layouts
An unclosed div is the single most common structural error. Because div is a block-level container, one missing closing tag can silently absorb every sibling element that follows it, collapsing the layout. This validator flags each unclosed div with its line number so you can locate and fix it directly, acting as a dedicated html div checker.
HTML structure checker, div close checker & html tag checker — three tools in one
This page works as a complete html structure checker: run it as a div close checker to catch every div that opens but never closes, or as a plain html tag checker to verify that every tag in your markup — not just div — has a matching close. Each result links back to the exact line number in your pasted code.
Nesting rules
Block elements (div, p, section) cannot be nested inside inline elements (span, a, em). A p element cannot contain block-level children — p > div is invalid. ul and ol should only have li as direct children. table rows must be inside tbody, thead, or tfoot.
Required attributes and accessibility
Every img must have an alt attribute — empty string is valid for decorative images, but the attribute must be present for screen readers. The html element must have a lang attribute for assistive technology. a elements should have meaningful href values. input elements should have associated label tags.
Deprecated and obsolete elements
HTML5 removed several presentational elements: center, font, marquee, blink, strike, big, basefont. These still render in most browsers but are no longer valid HTML5 and should be replaced with CSS. Using them signals old code that may have other quality issues.