Validate HTML Structure — Error Checker & Code Validator
Paste HTML to check for unclosed tags, invalid nesting, missing attributes, deprecated elements, and accessibility issues. Instant score, no login.
StructureNestingAccessibilityDeprecatedFree · No login
// HTML Structure Validator
// Paste HTML code0 lines
// Validation results
□Paste HTML and click Validate HTML Structure
What HTML structure validation checks
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.
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.
FAQ — HTML structure validation
HTML structure validation checks that your markup follows correct rules: tags are properly closed, elements are nested in valid parent-child relationships, required attributes are present, deprecated elements are not used, and the document has necessary structural elements (doctype, html, head, body, lang, charset). Valid HTML renders consistently across browsers and is easier for search engines to parse.
Most common errors: (1) Unclosed tags — a div or p opened but never closed. (2) Incorrect nesting — block elements like div inside inline elements like span. (3) Missing alt attributes on img elements. (4) Missing lang attribute on the html element. (5) Missing charset meta tag. (6) Deprecated elements (center, font, marquee). (7) Missing doctype declaration.
Valid HTML indirectly improves SEO. Googlebot uses a rendering engine based on Chromium which handles many HTML errors gracefully, but malformed structure can prevent correct DOM parsing of content. Missing lang attributes affect language detection. Unclosed tags can cause content to be excluded from Google's content extraction. Semantic structure (main, article, nav, h1-h6 hierarchy) helps search engines understand page structure.
This tool uses the browser's built-in DOMParser to parse the HTML string and then runs a series of checks on the resulting DOM tree: scanning for void elements, checking attribute presence, detecting deprecated tag names, and analyzing the document structure. All processing happens in your browser — nothing is sent to a server. The W3C validator uses the official Nu HTML Checker running server-side for more comprehensive spec-level validation.
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