Materialize CSS is one of my favourite CSS constructor for building pages prototypes.
It contains (comparing to bootstrap) all things, that might be needed for developer (at least I was missing select with stylish options, always third-party tool). And it is Google driven!
I used to have
redactor in all my projects, but I don't like how it works now with current CSS engine and it is.. commercial. My searches guided me to this
tiny boy and further, - to
It has few little glitches, but easy to have them fixed (i.e. layouts glitches with z-index levels, callbacks for image upload). Bring your comments below if you need more details.