HTML Code Validator
Find common HTML syntax issues instantly: unclosed tags, duplicate IDs, missing required attributes, deprecated tags, and unused CSS selectors. Your code stays in the browser.
Practical Example
Paste a static HTML page before deployment to find unclosed tags, duplicate IDs, missing image alt text, and unused CSS selectors. These issues may not be obvious in the browser but can affect SEO, accessibility, and cross-browser rendering.
What Is the HTML Code Validator?
Browsers silently repair many HTML mistakes, so broken markup can appear to work while still causing SEO, accessibility, or layout problems. This tool focuses on the mistakes developers run into most often while editing templates and snippets.
Paste a full HTML document or a fragment and run the check. All parsing happens locally in JavaScript, so internal code is not uploaded anywhere.
Detected Issue Types
Errors are structural problems that can change rendering, such as unclosed tags, tag mismatches, duplicate IDs, and missing image sources.
Warnings cover standards and accessibility problems, such as missing image alt text, anchors without href, deprecated tags, missing document title, and unused CSS selectors.
Info highlights quality improvements, such as empty elements, missing input/button type attributes, missing viewport meta, and missing doctype in full documents.
Unused CSS Selector Detection
The validator checks selectors defined inside inline <style> tags and reports selectors that do not match the pasted HTML. It also walks simple @media, @supports, and @container blocks. External stylesheets are not downloaded or analyzed.
Limitations
This is not a full W3C validator. It is designed for quick, local checks of common production mistakes. For formal standards validation, use validator.w3.org.
Most Frequent Real-World Errors and Their Cost
These are the errors this validator finds most often in static sites, and what actually goes wrong if they ship.
| Error | On screen | Actual consequence |
|---|---|---|
Unclosed <div> | Looks fine | Later elements adopt the wrong parent — CSS inheritance and selectors misfire |
Block element inside <p> | Looks fine | Browser force-closes the <p> — JS parent traversal breaks |
| Duplicate IDs | Looks fine | getElementById, anchors, and label bindings hit only the first match |
| Images without alt | Looks fine | Screen readers announce the file name; image SEO suffers |
| Malformed table markup | Mostly fine | Recovery differs per browser — layout breaks only in some environments |
The common thread: everything "looks fine" because browser error recovery hides the damage, while search engines, assistive tech, and other browsers each interpret the broken markup their own way. Item-by-item criteria are in the HTML validation guide.
FAQ
Does this tool upload my HTML?
No. The validator runs entirely in the browser and does not send your pasted HTML to a server.
Should I paste a full document or a snippet?
Both work. A full document enables checks for doctype, title, viewport, and html lang. A snippet focuses on tag structure, IDs, attributes, and inline CSS.
Does it analyze external CSS files?
No. It only checks CSS inside the pasted HTML's <style> tags.