Here is the complete list of tools I use when I design in the browser using css and html.
I try to write as little css as possible at this point. Tachyons is normally my base for putting something together. It handles most of what I need, is easy for me to extend and weighs less than 11kb. I generally strip out things that aren't needed for a project to get the weight down even more.
For processing and manipulating CSS files. I use the following modules:
This is my favorite way to manage styles. Encapsulating your styles in js will help reduce debugging time and you only have to ship the css you need for a view which can be a huge performance boost.
React has transformed my ability to iterate through designs and test different ideas out.
For auto-updating the browser after changing code.
For managing build flow and dependencies.
A Web Browser
For visual testing my css is working. I normally test on lynx, safari, chrome, and firefox as well as mobile iOS and chrome.
For writing HTML/CSS/JS.
For visualizing various stats about the css I'm refactoring.
Pen & Paper
I still use pen & to write down notes and sketch out design ideas. When I'm building a complex component I generally write down a list of things I know about the component and a list of things I don't know for sure.