Do you know about box-sizing: border-box; ? I didn't read your code all that carefully but I have a feeling that might come in handy.

Oh and you should always specify the base parameter for parseInt() otherwise you'll get unexpected values if the strings you're parsing have leading zeroes.

I wasn't aware of it (border-box). It looks like it could be very useful. Haven't had any problems with parseInt in my very limited use of it.

it was just a heads-up really – if you do

parseInt("0034")

for example, you'll get 28 as a result, whereas if you do

parseInt("0089")

you'll get either 0 or NaN.

In browsers which support ECMAScript 5, which is basically just Chrome at the moment,  this silly behaviour is removed and parseInt works how you'd expect. Still, best to be on the safe side.