CSS Diagnostics is a free Chrome extension developed by scriptex. It falls under the category of Browsers and is classified as an Add-ons & Tools subcategory. The main purpose of this extension is to highlight potentially invalid and erroneous HTML markup.

CSS Diagnostics utilizes a fork of the Holmes stylesheet, which can be accessed on the GitHub repository of redroot. By integrating this stylesheet, the extension is able to detect and bring attention to any HTML markup that may be incorrect or flawed. This can be particularly useful for web developers and designers who want to ensure the quality and accuracy of their HTML code.

With CSS Diagnostics, users can quickly identify any potential issues in their HTML markup, allowing them to make necessary corrections and improve the overall quality of their web pages. The extension provides a straightforward and accessible way to enhance the development process and ensure the integrity of HTML code.