I just open Sublime Text (a basic text editor) and start typing away HTML/CSS until I've got something I like the look of. No fancy IDE features or anything, just typing out code into a file :).

I also use my browsers' DevTools for faster CSS iteration, and Paint.NET for concept art and comparing/measuring screenshots. Sometimes I even use Illustrator and/or Inkscape for some SVG stuff (which I manually edit in code afterwards) - in this blogpost I used Inkscape to recreate the cat emoji and the Chrome iframe error pixel art icon.