HTML Validator

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.