I have been working on a grid design framework for able2know, and have been wrestling with the inconsistencies between browsers when it comes to percentage rounding. There are good static grid CSS frameworks out there (e.g. Blueprint, 960gs) but the attempts to convert them into liquid grids (e.g. Liquid Blueprint, fluid960gs) share the same bugs I was running into, where width is not correctly calculated and in IE6 and IE7 the effect was often dramatic, with columns jumping around when the browser is resized. At some widths everything would be perfect, but at others columns would wrap into the next row, breaking the layout. A couple of pixels off might not sound like much, but have a look at this video to see the effect on the layout those few pixels can have.
I set out to fix this problem, and began writing my own grid CSS framework, trying all sorts of different mathematical approaches to the problem (I was working with three columns so my initial suspicion was that the browsers were having a hard time dealing with fractions like 1/3). I also tried every IE CSS hack to try to coax the floats the grid uses into compliance, but no dice. So after more investigation, I learned that this is a rendering problem with no perfect solution for the browser manufacturs. The limitation is your monitor.
To make a very long story short, your monitor needs your browser to display objects sized in pixels. Browsers have no easy way around this problem, and there is no perfect solution. This, of course, doesn’t mean that the various browsers will find a consistent way to handle it, and as per usual IE managed to settle on the least optimal approach of the bunch.
First of all, let’s explain the problem. Let’s say you want to divide your layout into 4 columns. The pixels this area will represent may not be divisible by four. Let’s say you are dividing 50 pixels (no matter what you choose, the browser may end up with an indivisible problem, so I’m just using an easy example that John Resig used in his comparison of how each browser handles the rounding), each column should be 12.5 pixels wide according to your CSS telling the browser to divide the 50 pixels into 25% columns. The problem is that your browser can’t do this, and needs to round to an integer. No matter what your browser does at this point, there will be imperfections. If your browser rounds down there may be gaps and if you round up, then there can be an overflow.
Here is a test you can use to see this error in effect in your browser. It should display a black box, but drag your browser around resizing it, and at some sizes you should see gaps (in Firefox 3 you may not be able to do so due to an innovative way they are handling the rounding rounding the layout to 1/60th of a CSS pixel).
Now I understand the limitations each browser faces, and how they can’t possibly get it all perfect, but what IE does that breaks layouts as you resize the browser, causing your divs to jump around is round up. By rounding up they may cause your columns to exceed the width of their container, and wrap. There’s not a great way around this, and the only easy solution is to not let your columns add up to 100%, leaving room for rounding up without breaking your layout. It’s not a great solution, and it ruins the pixel perfect grid I was trying to implement on able2know with more space on the right than on the left but I thought I’d write up the problem in case others need it to fix their floats.