With Less, you can pare bootstrap down to just the bare minimum of what you need, all the way down to just using the grid an nothing else. When you compile your Less, it's super slim.

My focus wasn't on the bootstrap CSS customizer though, I try to recommend people use their own Less to keep things under control.

Min is an extremely small CSS framework (995 bytes) that has compatibility with Bootstrap but unlike Bootstrap isn't prescriptive in how you do things. It's easy to port sites to Min, and when you do the loading times will shock you. https://github.com/owenversteeg/min (Disclosure: I'm the author)

Min looks like a nice attempt at a lightweight framework, but it is so minimal that I am not sure I could ever find a use case for it. One could easily start with something like normalize and get something up and running just as quickly as implementing Min. I like Bootstrap because I can rapidly lay out an entire application interface in minutes. From there, I can digg into the LESS to customize components and remove the stuff I don't need. Sure, you end up with a larger CSS file, but if optimized it isn't much of an issue.

Well, Min 2.0 will be out soon (a few days) and will include everything that Bootstrap does and more. I guarantee that laying something out using only Normalize will be harder than using just Min. For one, Min is about the same size as Normalize and it supports more browsers (IE5.5+). It also provides buttons, a grid, a navbar, etc.

Try replicating Min's navbar quickly (the v2.0 one) and you will end up with tons of compatibility problems, strange bugs, and you'll give up quickly. There's a reason there are only five CSS frameworks (Bootstrap, Foundation, uikit, Ionic, Min) that actually have a working navbar. That's out of about 100 CSS frameworks total. All of those five frameworks, except Min, have a company, thousands of stars, and tons of contributors behind them.

I don't count Gumby as its navbar has some weird bugs, plus it uses display: table. I don't count Cascade and Ink as neither work consistently on Android.

For a preview of Min 2.0, try http://pasteht.ml/Rg0B0

I've been really happy with PureCSS[0], and it seems to be gaining quite a bit of traction lately (over 8000 stars on GitHub). PureCSS is very small and modular. If I didn't like PureCSS so much, I would probably roll my own grids using Bourbon Neat or Susy.

0 - http://purecss.io/