HTML Validator

Beautify
Minify
Validate
Compare

Input HTML

0 chars | 0 lines

Output HTML

0 chars | 0 lines

About the HTML Validator

Here's something I learned the hard way: invalid HTML doesn't crash browsers — it silently gets "fixed" by the browser's error-recovery algorithm. The problem is every browser fixes it differently. A missing <tbody> that Chrome auto-inserts might be placed differently in Safari, causing layout shifts you can't reproduce in your dev environment.

This validator catches those issues before they reach production. It checks for structural problems — unclosed tags, wrong nesting, deprecated attributes, duplicate IDs — and explains each issue clearly. I designed the error messages to tell you not just what is wrong but why it matters.

Features

Structural validation — unclosed tags, improper nesting, missing required attributes
Duplicate ID detection — catches a common JavaScript bug source
Deprecated element warnings — flags <center>, <font>, <marquee> etc.
Accessibility hints — missing alt on images, missing form labels
Inline validation — errors shown with line references
Works offline — entirely browser-based, no W3C API call

The 7 Most Common HTML Errors (From Real Code)

After validating thousands of HTML files through this tool, these are the errors that appear most often:

Unclosed tags — especially <div>, <li>, and <td>
Duplicate id attributes — breaks document.getElementById() silently
Missing alt on <img> — accessibility and SEO failure
Block elements inside inline elements — e.g., <div> inside <span>
Missing <!DOCTYPE html> — triggers quirks mode in IE/Edge
Deprecated attributes — align, bgcolor, border on table elements
Invalid attribute values — non-numeric width, invalid href

Use this alongside the HTML Beautifier — beautify first to make the code readable, then validate to fix the errors, then minify for production with the HTML Minifier.

Client-Side vs. W3C Validation

The W3C validator is the gold standard but requires a network request and doesn't work offline. Our validator is intentionally client-side — it's fast, private, and available without internet. For production deployments, we recommend running both: use ours during development for instant feedback, and run the W3C validator as a final check before launch.

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