HTML Validator

What Is an HTML Validator?

An HTML validator inspects your markup and reports problems that browsers, screen readers, and search engines may handle differently than you expect. This HTML validator runs entirely in your browser and checks for the mistakes that show up most often in real projects: unclosed tags, mismatched closing tags, duplicate IDs, duplicate attributes, images without alt text, links without destinations, form controls without labels, deprecated elements, and skipped heading levels.

Every issue includes the line number where it was found, so you can jump straight to the problem instead of hunting through hundreds of lines of markup. Results update as you type, which makes the validator useful during development rather than as a final gate before shipping.

Common HTML Problems This Tool Catches

• Unclosed and mismatched tags. A missing </div> or a <span> closed with </div> leaves the document structure ambiguous. Browsers silently repair these errors, but they do not all repair them the same way.
• Stray closing tags. A closing tag with no matching opening tag is always ignored, and often signals a copy and paste mistake.
• Duplicate IDs. IDs must be unique. When they are not, getElementById, anchor links, and label for associations resolve unpredictably.
• Duplicate attributes. Browsers keep the first occurrence and drop the rest, which can silently discard a class, href, or style.
• Images without alt text. Missing alt attributes break the experience for screen reader users and remove a source of image context for search engines.
• Links without destinations. An anchor with no href, an empty href, or href="#" is unreachable by keyboard navigation in many browsers.
• Form controls without labels. Inputs, selects, and textareas that lack a label, aria-label, or wrapping label are hard to use with assistive technology.
• Deprecated elements and attributes. Tags like <center>, <font>, and obsolete attributes such as bgcolor still render in most browsers but are no longer part of the standard.
• Skipped heading levels. Jumping from an h2 straight to an h4 breaks the document outline that screen readers use to navigate.
• Missing document metadata. No lang attribute on the html element, no title, or no character encoding declaration.
• Self-closing non-void elements. Writing <div /> is treated as an opening tag in HTML, so the element is never closed.
• JavaScript URLs. javascript: hrefs are flagged because they are blocked by many content security policies and are a common injection vector.

Why Valid HTML Matters

Modern browsers are extremely forgiving. They will render markup that is technically invalid, and they will quietly repair many errors. The problem is that the repair rules are not identical across engines, and they are not something you control. A document that looks fine in one browser can produce a different DOM in another, which shifts layout, breaks scripts that query specific elements, or changes what a crawler sees.

Accessibility is less forgiving still. Screen readers depend on correct structure: proper heading order, labels tied to fields, and semantic elements instead of clickable divs. Validation is not a substitute for testing with assistive technology, but it removes a large class of avoidable barriers before testing.

For search engines, clean structure helps crawlers parse content accurately. Heading hierarchy signals topic structure, valid lists and tables make extraction more reliable, and correctly nested markup lowers the chance that important text is misread or ignored.

How to Use This HTML Validator

1. Paste or type your HTML into the editor. A sample document loads automatically so you can see the checks in action.
2. Read the summary cards for a quick count of errors, warnings, and elements checked.
3. Work through the issue list from top to bottom. Each entry shows severity, the line number, and a short fix suggestion.
4. Fix a problem and the list updates immediately. Aim for zero errors first, then clear the warnings that matter for your project.

Common Checks Versus Full Specification Validation

This tool focuses on the issues that cause real problems in practice. It is deliberately not a complete HTML5 conformance checker: it does not parse the full content model of every element, does not validate every attribute value against the specification, and does not verify ARIA usage in depth. For a standards conformance report before publishing a public page, also run the document through the W3C validator. Think of the two as complementary: this tool catches practical, high frequency mistakes instantly while you work, and the W3C validator provides the exhaustive standards check.

Privacy

All parsing and checking happens locally in your browser with the built in HTML parser. Your markup is never transmitted, stored, or logged, which makes the tool safe for proprietary templates, internal tools, and client work under NDA.

Related Tools

Continue your markup workflow with these tools:

• HTML Formatter
• HTML Minifier
• HTML Preview
• HTML Cleaner
• HTML to Markdown
• XML Validator