ZenovayTools

HTML Validator

Free HTML5 validator: checks DOCTYPE, lang attribute, title, meta charset, missing alt attributes, duplicate IDs, deprecated elements (<center>, <font>, <marquee>), unlabeled inputs. W3C Validator alternative.

How to Use HTML Validator

  1. 1Enter a URL to validate the live page, or paste your HTML directly into the text field.
  2. 2The tool checks for critical errors, warnings, and informational issues.
  3. 3Issues include missing DOCTYPE, missing alt attributes, duplicate IDs, deprecated elements, and more.
  4. 4A grade A-F reflects overall HTML quality and standards compliance.
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Frequently Asked Questions

What HTML5 rules does this validator check?▾
The validator checks: (1) DOCTYPE declaration — required for standards mode rendering. (2) lang attribute on <html> — required for WCAG 3.1.1 accessibility and correct language detection. (3) <title> element presence. (4) <meta charset> declaration. (5) <meta name="viewport"> for mobile. (6) Images missing alt attributes (WCAG 1.1.1). (7) Duplicate IDs — must be unique per HTML spec. (8) Deprecated HTML elements: <center>, <font>, <marquee>, <blink>, <frame>, <frameset>, etc. (9) Empty href attributes. (10) Form inputs without labels or aria-label.
How does this compare to the W3C Validator?▾
The W3C Markup Validation Service (validator.w3.org) does full DTD/schema-based parsing of HTML and reports precise syntax errors with line numbers. This tool focuses on the most impactful real-world issues: accessibility violations, SEO problems, deprecated elements, and security-relevant patterns (inline scripts without nonce). It's faster and works for any URL without browser restrictions. For production sites, both tools are complementary — use this for quick audits and W3C for strict compliance checking before launch.
Why is the lang attribute required on the <html> element?▾
The lang attribute (e.g., <html lang="en">) is required for: (1) Screen readers to use the correct text-to-speech voice. (2) Browser spell-checking and auto-correct to work correctly. (3) CSS :lang() selectors and language-specific typography. (4) Search engines to serve your page to users in the right country. (5) WCAG 2.1 Success Criterion 3.1.1 Level A — failing this is an accessibility violation. Use BCP 47 language codes: "en", "de", "fr", "es", "zh-CN", etc.
What are the most common HTML validation errors?▾
The most common issues are: (1) Missing alt on images — affects screen readers and image SEO. (2) Missing or wrong DOCTYPE — causes quirks mode rendering in some browsers. (3) Missing <meta charset="UTF-8"> — can cause garbled text for non-ASCII content. (4) Duplicate IDs — causes JavaScript to find the wrong element and breaks anchor links. (5) Deprecated elements like <center> and <font> — browser support is dropping. (6) Missing <title> — critical for SEO and browser tabs. (7) No viewport meta — site breaks on mobile.
Should I validate HTML for SEO purposes?▾
Google has stated it can parse broken HTML, but valid HTML correlates with better SEO for several reasons: (1) Missing alt text directly hurts image SEO and indexing. (2) Duplicate IDs can confuse structured data parsers. (3) Invalid nesting can cause layout issues that hurt Core Web Vitals. (4) The <title> and meta description directly affect CTR in search results. (5) Missing lang attribute affects international SEO and hreflang functionality. Valid HTML is a signal of technical quality that indirectly helps rankings.