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HTML Validator & Syntax Checker ⚡ W3C Valid

Validate HTML markup, catch unclosed tags, syntax errors, duplicate IDs, and accessibility issues with an instant quality score.

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Why Validate Your HTML5 Markup?

Browsers automatically attempt to patch broken HTML under the hood, but those silent auto-fixes frequently trigger layout shifts, broken accessibility trees, and indexing issues for search engine crawlers. A single unclosed <div> or mismatched quote can break your page's responsiveness across different devices.

This validator analyzes your raw source markup character-by-character to catch unclosed tags, illegal nesting, duplicate IDs, missing image alt attributes, and deprecated HTML elements. All validation runs 100% inside your browser memory with zero network uploads.

How to Validate HTML & Fix Common Errors

  1. Paste or Upload Markup: Copy your HTML source code into the left editor pane, or click Upload File to load an existing .html file from your device.
  2. Instant Quality Score & Summary: Review the overall Quality Score (0–100) and letter grade (A+ through F) in the validation card, highlighting the exact count of critical errors, warnings, and best-practice notices.
  3. Filter by Severity: Use the segmented filter tabs (Errors, Warnings, Notices) to focus on urgent blockers like unclosed tags or duplicate IDs first.
  4. Review Context & Suggested Fixes: Each issue card shows the precise line and column number, a preview of the offending code snippet, and actionable guidance on how to fix the error.
  5. Copy Report or Format: Click Copy Report to export a structured text report for code reviews, or click Format HTML → to automatically beautify and indent your validated markup.

HTML Validation Rules & Severity Matrix

Below is the reference matrix of checks performed by the validation engine, detailing why each issue matters and how it impacts your users and SEO.

Rule / Check Severity Real-World Impact Standard Remedy
Unclosed Tag Error Cascades layout destruction, breaks Flexbox/Grid containers, and causes elements to bleed into neighbors. Add matching </tag> before parent closes
Mismatched Nesting Error Tags closed out of order (e.g. <b><i></b></i>) corrupt the DOM tree and fail XHTML/XML compatibility. Close inner tag before outer tag
Duplicate ID Error Causes getElementById() to return only the first match, breaks aria-labelledby, and breaks CSS #id selectors. Use unique IDs or switch to class="..."
Missing <img alt> Warning Violates WCAG 1.1.1 accessibility guidelines; blind/visually impaired users cannot understand image content. Add alt="Description" or alt="" if decorative
Missing DOCTYPE Warning Triggers legacy Quirks Mode rendering in modern browsers, changing box model math and font calculations. Prepend <!DOCTYPE html> at line 1
target="_blank" Without rel Warning Enables tabnabbing security vulnerability where the newly opened site can redirect your tab via window.opener. Add rel="noopener" or rel="noreferrer"
Deprecated Element Warning Obsolete elements (<center>, <font>, <marquee>) are dropped from modern specs and ignore CSS responsive layouts. Replace with modern CSS styling
🌐 Need to format, indent, or minify your HTML?

Once your markup is validated, beautify its indentation or minify it for production using our HTML Formatter.

Try HTML Formatter & Minifier →

Frequently Asked Questions

How does this HTML Validator detect unclosed tags? ▼
Is my HTML code uploaded to any third-party server? ▼
Why is duplicate ID detection critical in HTML5? ▼
Can I validate an uploaded HTML file? ▼
How is the HTML Quality Score calculated? ▼
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