An HTML5 validator checks your markup for structural errors, missing required attributes, deprecated tags, and accessibility issues. Valid HTML renders consistently across browsers, improves SEO, and makes pages accessible to screen readers — paste your HTML below to catch issues before they reach production.
How to Validate HTML5
Validating HTML catches problems that cause inconsistent rendering, break screen readers, hurt SEO, and create cross-browser compatibility issues. This HTML5 validator uses the browser's built-in DOMParser API to check your markup for common errors without uploading anything to a server.
Step 1: Paste Your HTML
Paste the HTML you want to check into the input area. You can paste a full page with DOCTYPE, a component fragment, or any HTML snippet. To validate a live page, open its source in your browser (Ctrl+U / Cmd+U), copy all, and paste here.
Step 2: Click Validate
Click the Validate HTML button. The tool runs a series of checks covering: missing
alt attributes on images,
deprecated tags like <font> and
<marquee>,
missing DOCTYPE, missing lang on
<html>, duplicate
id attributes, and more.
Step 3: Review Errors, Warnings, and Info
Results are grouped by severity. Errors (red) are standards violations or broken accessibility that must be fixed. Warnings (amber) are non-critical but strongly recommended improvements. Info (blue) are best-practice suggestions.
What the Validator Checks
The validator runs these specific checks: missing alt
on images, missing href on anchor tags,
form inputs missing id or
name, deprecated tags (font, center,
marquee, blink, strike, tt), missing DOCTYPE, missing <title>, missing
lang on <html>, duplicate
id values, and empty
alt attributes on non-decorative images.
FAQ
Is this HTML5 validator free?
Yes, completely free with no account, no limits, and no usage caps. Validate as much HTML as you need.
Is my HTML code kept private?
Yes. All validation runs entirely in your browser using DOMParser, a built-in browser API. Your code is never sent to any server, stored, or logged.
What errors does this validator detect?
The validator checks for missing required attributes (alt on img, href on a), deprecated HTML4 tags (font, center, marquee), missing doctype, missing lang on html, duplicate IDs, unclosed tags, and basic nesting issues.
What is the difference between an error and a warning?
Errors are issues that break standards compliance or accessibility — for example, an img tag without an alt attribute. Warnings are non-critical but recommended fixes — for example, an empty alt attribute on a potentially non-decorative image. Info messages are best-practice suggestions.
Why are deprecated tags flagged?
Tags like font, center, marquee, and blink were removed in HTML5 and replaced by CSS. Modern browsers may still render them, but they indicate outdated code and create maintenance headaches. A validator flags them so you can update to modern equivalents.
Does this replace the W3C HTML validator?
For everyday HTML checking, this tool catches the most common issues quickly. The W3C Nu HTML Checker (validator.w3.org) performs a more exhaustive spec-level check and may catch edge cases this tool does not. Use both for mission-critical pages.
What does 'missing lang attribute' mean?
The lang attribute on the html element tells browsers and screen readers what language the page is written in. Missing it is a WCAG 3.1.1 accessibility failure. Fix: add lang='en' (or the appropriate language code) to your opening html tag.
How do I check a full web page's HTML?
Use your browser's View Source (Ctrl+U or Cmd+U), select all, copy, and paste into the validator. For live pages, browser DevTools (Elements panel) can also export the DOM as HTML.