Does this send my markup anywhere?

No. Validation runs entirely in your browser.

Is this the same as the W3C Markup Validator?

No. This checks structural well-formedness (matched tags, valid nesting) using Prettier's HTML parser, not full W3C spec conformance (deprecated attributes, ARIA rules, etc.) the way validator.w3.org does.

Does it check accessibility or SEO issues?

No. It's a structural well-formedness check only; it doesn't audit ARIA attributes, heading order, alt text, or other accessibility and SEO concerns.

Can I validate a JSX or templating file (Vue, Handlebars) with this?

Not reliably. It uses Prettier's plain HTML parser, which doesn't understand JSX expressions or template-engine directives like {{ }}, so those can trigger a false failure even in a file that's otherwise correct.

HTML Validator - Check HTML Syntax Online

Overview

Introduction

Browsers are famously forgiving about malformed HTML, silently correcting problems that can still cause subtle rendering bugs.

This tool checks HTML structural well-formedness entirely in your browser, catching the mismatched or unclosed tags a browser would otherwise paper over.

What Is HTML Validator?

A well-formedness checker that parses your markup with Prettier's HTML parser and reports success or the first parse error.

It's a structural check (matched tags, valid nesting), not a full W3C spec conformance check.

How HTML Validator Works

The parser attempts to build a tree of elements from your markup.

If parsing succeeds, the markup is well-formed; if it fails, the exact line and column of the problem is shown.

When To Use HTML Validator

Use it when debugging unexpected rendering and you want to rule out a structural markup problem before checking CSS or JavaScript.

It's also useful for a quick pass/fail check on LLM-generated or copy-pasted HTML before adding it to a project.

Often used alongside HTML Formatter and HTML Minifier.

Features

Advantages

Runs entirely client-side.
Catches structural issues a lenient browser parser would silently paper over.
Uses the same Prettier HTML parser as the HTML Formatter, so a 'valid' result is one you can trust to also format cleanly.

Limitations

Doesn't check full W3C spec conformance (deprecated attributes, accessibility rules); use validator.w3.org for that.

Examples

Mismatched tags

Input

<div><p>Hello</div></p>

Output

Unexpected closing tag "div"

The parser reports the point where a closing tag didn't match the currently open element.

Best Practices & Notes

Best Practices

Use this before the HTML Formatter when you specifically want a fast pass/fail structural check without reformatting anything.
If validation fails on a template file (JSX, Handlebars, Vue SFC), strip the templating syntax first, since only plain HTML is supported.

Developer Notes

Reuses the same Prettier `html` parser as the HTML Formatter; the input is discarded on success and only the parse result matters.

HTML Validator Use Cases

Ruling out a structural markup problem when a page renders unexpectedly
Checking a generated or LLM-produced HTML snippet before using it

Common Mistakes

Assuming a 'valid' result means the page passes full W3C or accessibility validation; it only confirms basic well-formedness.
Pasting a JSX or templating-engine file directly; curly-brace expressions and directives aren't standard HTML and can trigger a false failure.

Tips

For full spec and accessibility validation, follow up with the W3C Markup Validation Service.
If a specific tag is flagged unexpectedly, check for a void element written with a mismatched closing tag, like </br>.

HTML Validator