HTML Validator

HTML validation checks your markup against the HTML5 specification to identify syntax errors, structural problems, and accessibility issues. Common validation errors include: unclosed tags that leave the DOM structure ambiguous, mismatched tags (opening a div but closing a span), deprecated attributes that modern browsers may not support, missing required attributes (img without alt, form inputs without labels), nesting violations (block elements inside inline elements, interactive content inside other interactive content), and duplicate IDs which cause JavaScript getElementById to behave unpredictably. Invalid HTML often renders correctly in modern browsers due to their error recovery algorithms — browsers silently fix many markup errors. However, this error recovery is not standardized, meaning different browsers may interpret the same invalid HTML differently, creating cross-browser inconsistencies. Screen readers and assistive technologies are less forgiving of invalid HTML — structural errors can make content inaccessible. Search engine crawlers may misinterpret invalid HTML, potentially missing content or misunderstanding page structure. The AllTools HTML Validator checks your markup in the browser, reporting errors with line numbers and suggestions for fixes. Your HTML stays on your device.

Valid HTML is a foundation for both web accessibility and search engine optimization. Accessibility standards (WCAG 2.1) require proper HTML structure — heading hierarchy (h1 followed by h2, not h1 followed by h4), form labels associated with inputs, meaningful alt text for images, ARIA roles on interactive elements, and semantic landmark elements (nav, main, footer) for screen reader navigation. The HTML Validator identifies these structural issues before they become accessibility barriers. For SEO, valid HTML helps search engine crawlers parse page content accurately — semantic heading structure signals content hierarchy, properly nested lists indicate related content, and valid table markup enables rich snippet extraction. Structured data (JSON-LD, Microdata) embedded in HTML must be valid to generate rich search results. Email HTML has its own validation concerns — email clients support a limited subset of HTML and CSS, and invalid markup produces wildly different renders across Outlook, Gmail, Apple Mail, and mobile clients. Validating email HTML catches issues before they reach subscribers. The AllTools validator provides immediate feedback for iterative development without requiring you to deploy code to a staging server for testing.