#096 · HTML Developer Tool

HTML Validator

Validate HTML syntax and document structure without uploading source code. The validator checks tag balance, invalid nesting patterns, duplicate IDs, deprecated elements, required document metadata, and common attribute problems, then reports line-oriented findings.

Developer Input

HTML validation settings

HTML source

<!doctype html>
<html lang="en">
<head><meta charset="utf-8"><title>Validation example</title></head>
<body><main><h1>Example</h1><p>Valid paragraph.</p><img src="example.jpg" alt="Example"></main></body>
</html>

Validation level

Maximum issues

Issues

Trim outer whitespace Include warnings Run practical checks Auto-run while typing

How to use this developer tool

Paste or type the source content in the main editor.
Choose the processing mode and adjust the tool-specific option.
Enable or disable the validation and live-processing options as needed.
Select the primary action, review the report and statistics, then copy or download the result.

What this developer tool does

Validate HTML syntax, tag balance, nesting, duplicate IDs, deprecated elements, metadata, and common accessibility attributes.

Scan source tags → parse DOM → inspect nesting, IDs, attributes, deprecated elements, and document essentials.

All processing occurs locally in the browser. Review automated findings before using the result in production.

Example

The editor is preloaded with a working HTML Validator example. Change the sample, select an option, and run the tool to compare the input with the generated output and report.

Tips for better output

Validate a complete document for metadata checks.
Confirm reported issues in the relevant HTML specification for edge cases.
Fix errors before warnings.

FAQ

How do I validate HTML5 without uploading a file?

Paste the HTML and run the practical HTML5 validation profile. All checks run locally in your browser.

Why does an HTML validator report duplicate IDs?

HTML id values must be unique within the document. Duplicate IDs can break labels, fragments, scripts, and accessibility relationships.

Can this validator find missing closing tags?

It performs source-level tag-balance checks and DOM-level structure checks for common missing or mismatched tags.

Does HTML validation include deprecated tags?

Yes. Common deprecated elements such as font, center, marquee, and frameset are reported.

Can I validate accessibility-related HTML attributes?

The accessibility profile checks items such as image alt text, form labels, button names, document language, and heading structure.

Related developer tools

HTML Checker HTML Generator HTML Converter HTML Parser

HTML Validator modules

Module	Details
Syntax	Tag balance and malformed source
Structure	Nesting and duplicate IDs
Standards	Deprecated elements and essentials
Output	Line-oriented report

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