The W3C Markup Validation Service is a free online tool from the World Wide Web Consortium (W3C) that lets you check whether the HTML or XHTML code of a web page is formally correct. It analyzes the source code and displays errors, warnings, and suggestions to help keep web pages technically clean and standards-compliant.

The W3C CSS Validation Service is a free online tool from the World Wide Web Consortium (W3C) that lets you check CSS stylesheets for errors, warnings, and standards compliance.

It helps web developers write clean, compatible, and future-proof CSS code.

Function

Markup Validator

W3C Markup Validation Service checks HTML, XHTML, SMIL, MathML, and other markup languages ​​for valid syntax and helps improve technical quality and browser compatibility.

CSS Validator

Checks external CSS files, inline CSS, and embedded CSS in HTML documents and reports syntax errors, deprecated properties, incorrect values, and warnings. Various CSS profiles (e.g., CSS Level 3) and media types are supported.

{Note regarding HTML5: The classic validator is based on DTDs, which HTML5 no longer uses. For modern HTML5 pages, the W3C now uses the Nu HTML Checker, which is also available online. [1]}