A fork of the Bootstrap 3 CSS framework (formerly "Twitter bootstrap"), with an additional break point at 480px. Completely backward compatible.

The original Bootstrap 3 ships with break points at 0px (xs), 768px (sm), 992px (md) and 1200px (lg).
There was a long discussion on the bootstrap issue queue about adding a new break point at 480px, to distinguish between narrow-screen mobiles, and bigger mobiles or tablets.

The problem here is, how could this be done without hurting backwards compatibility?

The xs-AB-subdivision branch on the donquixote bootstrap 3 fork introduces a solution that is completely backward compatible.
Existing known bootstrap classes like col-xs-6 etc don't change their meaning.
Instead, new subdivision classes are introduced, which can be used for more fine-grained scaffolding.

Currently only the first range of 0px - 767px ("xs") is subdivided once: 0px - 479px (xs0) and 480px - 767px ("xs1").
But more subdivision classes would be easy to add: sm0 vs sm1, md0 vs md1, lg0 vs lg1.
This could even go more fine-grained, by added another level of subdivision: xs00, xs01, xs10, xs11, etc.

This bootstrap fork is currently successfully used on Gruenberg E-Bikes and Avian Bike.
It is very soon going to be used on Teakmoebel.com and E4BTC (Electronics for Bitcoin).