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  1. The W3C Markup Validation Service

    This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. If you wish to validate specific content such as RSS/Atom feeds or CSS stylesheets, MobileOK …

  2. Free Online HTML Validator - FreeFormatter.com

    This free online HTML validator lets you validate your files against the w3c standards

  3. JSON Checker - The JSON Validator and Formatter

    A quick json validator with beautiful syntax highlighting and easy to understand error messages

  4. JSONLint - The JSON Validator

    JSONLint is an online editor, validator, and formatting tool for JSON, which allows you to directly type your code, copy and paste it, or input a URL containing your code.

  5. validator.w3.com

    Online tool for validating web documents against W3C standards.

  6. HTML Validator - Check & Validate HTML Code Online

    The HTML Validator is a tool we created to help you write clean and correct HTML code. When building a webpage, it's easy to make a small mistake, like forgetting to close a tag or using a duplicate ID.

  7. HTML Validator - HTMLStrip

    It is a simple free HTML validator which is based on specifications provided by w3.org. This tool provides guidance on markup errors and best practices on Web documents in HTML, XHTML, SMIL, MathM …