@Rachel Re: the concept of fractional pixels. While they may not physically exist, they are digitally recreated through transparencies and color variations. A prime example of this can be seen in icons and type serifs. Upon closer inspection, what seems like a thin line is actually mimicked by rendering the line with a lighter color to deceive the eye. Graphic software has effectively handled fractional pixels for quite some time now, even if they are technically non-existent. It's unfortunate that webkit browsers have yet to catch up.
Regarding fractional letter spacing, it can be defined and functions well in non-webkit browsers (good job, IE). In webkit browsers, however, letter spacing is rounded off to the nearest whole number (usually rounding down). This rounding occurs not only when the letter-spacing is directly specified as a fractional pixel but also when it is set as a percentage or em value, leading to a final calculation in pixels resulting in a fraction. This can be incredibly frustrating for developers and designers alike.