Validate any URL against the W3C HTML5 spec. Find every parser error, stray tag, duplicate ID, and accessibility-blocking issue in seconds.
HTML validation is one of those things that sounds pedantic until it bites you. Browsers are extraordinarily forgiving — they'll render almost any malformed HTML you throw at them — but everything ELSE that consumes your HTML is not. Screen readers, search engine crawlers, RSS aggregators, scraping tools, and accessibility audits all interpret your markup strictly. This tool runs your URL through the W3C's official Nu HTML Checker (the same one nu.validator.w3.org uses) and shows you every error, warning, and info message.
## Why "my page renders fine" is not the same as valid
Chrome's parser implements HTML5 error recovery — a 60-page spec dedicated to guessing what you meant when your HTML is broken. It usually guesses right, which means you ship broken HTML and never notice. But Googlebot, lighter-weight crawlers, and assistive tech don't implement the full error-recovery spec. Where Chrome guesses, they fail silently or skip content. The result: a page that looks fine but ranks poorly and fails accessibility audits.
## The duplicate-ID disaster
The single most common HTML validation issue we see is duplicate `id` attributes. They're benign-looking — a CMS template generates `<section id="hero">` and then later your sidebar also has `id="hero"`. The page renders. JavaScript breaks in non-obvious ways: `document.getElementById("hero")` returns only the first match, so any code that targets the second one silently does nothing. ARIA attributes referencing the duplicate `id` (like `aria-labelledby="hero"`) point at the wrong element. Screen readers announce the wrong content.
## What the Nu Checker actually checks
It implements the full HTML5 spec — element nesting rules, attribute value formats, ARIA validity, microdata, form input constraints, and more. It does NOT check whether your CSS or JavaScript works, and it does NOT check accessibility beyond what the spec mandates (you need axe or Lighthouse for full a11y audits).
## Should I aim for zero errors?
Yes for new pages. For legacy pages, focus on errors that change behaviour: duplicate IDs, missing `alt`, malformed ARIA, missing `lang`, missing form labels. Stylistic warnings ("trailing slash on void element") are safe to ignore unless you're building a strict-mode project.
## How errors map to SEO impact
Most HTML errors don't directly affect SEO — Googlebot uses Chrome's parser. But: missing `lang` blocks language detection. Missing `alt` blocks image search ranking. Duplicate IDs can break structured-data extraction (your Article schema may go undetected). Stray tags inside `<head>` can cause Google to truncate your `<title>` or `<meta description>`. Run this once per template, fix what matters, then re-run after any CMS migration.
Invalid HTML rarely breaks rendering (browsers are forgiving) but it routinely breaks accessibility tools, screen readers, parsers, and SEO crawlers. Duplicate IDs silently break JavaScript. Stray tags can hide content from Googlebot. The W3C Nu Checker is the same validator Google's own teams use to QA their pages.
We submit your URL to https://validator.w3.org/nu/?doc=…&out=json, the official W3C Nu HTML Checker JSON endpoint. The validator fetches your page server-side, parses it against the full HTML5 spec, and returns a list of messages classified as error, warning (info subType=warning), or info. We split them into errors, warnings, and infos for display, flag likely "fatal" errors (stray characters, unclosed tags, duplicates) separately, and link out to the live validator UI so you can drill into the full report. We cap stored errors/warnings at 50 each to keep responses fast. The validator itself is open source and used by Google's internal SEO teams — it's the same source of truth your search engine uses.
Chrome implements HTML5 error recovery — it guesses what you meant. The validator is strict; Googlebot, screen readers, and accessibility tools are closer to strict than Chrome.
Aim for zero on new pages. For legacy, fix errors that change behaviour (duplicate IDs, missing alt, missing lang, malformed ARIA). Stylistic warnings are safe to ignore.
Indirectly. Google parses with Chrome's forgiving parser, so most errors don't affect indexing. But: missing alt = no image search; missing lang = no language detection; broken structured data = no rich results.
No — the W3C validator fetches static HTML only. For SPAs, use the browser extension version that validates the live DOM after JS runs.
Errors are spec violations that change semantics. Warnings (technically "info with subType warning") are best-practice nudges that won't break rendering or parsing.