# ValidateHTML

> Free online validators and developer tools for HTML, CSS, JSON, and XML, plus SEO and accessibility checkers. Paste your code or enter a URL to get errors, warnings, and a 0-100 quality score with letter grades. No registration, no limits.

ValidateHTML runs spec-accurate engines (html-validate, csstree, fast-xml-parser) in a fast, modern interface. Every tool is free and processes code in memory without storing it.

## Validators
- [HTML Validator](https://validatehtml.com/): Validate HTML against modern standards with a quality score and detailed error reports.
- [HTML5 Validator](https://validatehtml.com/validate-html5): Validate against the latest HTML5 specification; catch deprecated elements and spec violations.
- [CSS Validator](https://validatehtml.com/css-validator): Check CSS for syntax errors, invalid properties, and incorrect values.
- [JSON Validator](https://validatehtml.com/json-validator): Validate and format JSON data with syntax error detection.
- [XML Validator](https://validatehtml.com/xml-validator): Check XML files for well-formedness and syntax errors.

## Formatters
- [HTML Formatter](https://validatehtml.com/html-formatter): Beautify or minify HTML with customizable indentation.
- [JSON Formatter](https://validatehtml.com/json-formatter): Format and pretty-print minified or messy JSON.

## SEO Tools
- [Meta Tag Checker](https://validatehtml.com/meta-tag-checker): Analyze title, description, viewport, Open Graph, and Twitter Card tags.
- [Open Graph Checker](https://validatehtml.com/open-graph-checker): Preview how a page looks when shared on social media.
- [Robots.txt Checker](https://validatehtml.com/robots-txt-checker): Test which pages are blocked from crawlers.
- [Page Size Checker](https://validatehtml.com/page-size-checker): Analyze page weight, compression, and load time.
- [Accessibility Checker](https://validatehtml.com/accessibility-checker): Audit alt text, heading hierarchy, ARIA, and WCAG compliance.

## Error Guides
- [HTML Errors](https://validatehtml.com/html-errors): Common HTML errors and how to fix them, with wrong vs correct examples.
- [CSS Errors](https://validatehtml.com/css-errors): Common CSS mistakes and fixes.
- [JSON Errors](https://validatehtml.com/json-errors): Common JSON syntax errors and fixes.
- [XML Errors](https://validatehtml.com/xml-errors): Common XML errors and fixes.
- [Accessibility Errors](https://validatehtml.com/accessibility-errors): Common accessibility issues and fixes.

## References
- [HTML Entities Reference](https://validatehtml.com/html-entities-reference)
- [HTTP Status Codes](https://validatehtml.com/http-status-codes)
- [Robots.txt Syntax](https://validatehtml.com/robots-txt-syntax)
- [Viewport Meta Tag Guide](https://validatehtml.com/viewport-meta-tag)
- [Blog](https://validatehtml.com/blog)
- [All Tools](https://validatehtml.com/tools)