CSS HTML Validator (previously named CSE HTML Validator) is an HTML editor and CSS editor for Microsoft Windows (and Linux and other Unix-like operating systems when used with Wine) that helps web developers create syntactically correct and accessible HTML/HTML5, XHTML, and CSS documents by locating errors, potential problems like browser compatibility issues, and common mistakes.

CSS HTML Validator is developed, marketed, and sold by AI Internet Solutions LLC located in Texas.

A free limited version of the htmlval command line tool for macOS and Linux is also available.

TNPL can also be used to integrate customized validation checks to meet the unique requirements of an individual or entity.

Version 18 also added a new "By Message" report in the Batch Wizard and dropped support for Windows Vista and below.

CSS HTML Validator 2022/v22, released in January 2022, includes improvements and updates to keep the program up-to-date, a new Microsoft Edge WebView2 rendering engine for the integrated web browser, and three new dark themes.

Later updates to version 22 added support for checking JSON Lines and NDJSON documents.

CSS HTML Validator 2023/v23, released in January 2023, includes more improvements and updates to keep the program up-to-date.

An online edition based on CSS HTML Validator Pro that can check documents via file upload, URL, or snippets (direct text input) was discontinued May 2017 in favor of the desktop version for Microsoft Windows.