draw.io - Online diagramming application
CSS Peeper - Smart CSS viewer tailored for Designers.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
Purgecss - Easily remove unused CSS
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.