Just looking through the list of issues for the chromium project, a large number of them are cross-platform text rendering issues. Having to use the platform's text rendering system, and cater to their respective idiosyncrasies, I imagine is a lot of work.
Why not simply use the FreeType library and be done with it? FreeType is used in Linux, iOS and Android, among other platforms. So on these platforms, browsers are using a wrapped FreeType library anyway. Why not scrap that wrapping, scrap the platform-supplied rendering on Windows and Mac, and go pure FreeType?
The only answer that occurs to me is that perhaps it's to take advantage of the hardware accelerated composition and blending of glyphs that Windows- and Mac-supplied rendering offers.
Or my other answer is that perhaps it was because WebKit was ported before the subpixel antialiasing patents had expired, so the only way to get subpixel antialiased text was to use platform-supplied rendering.
Are either of these correct? If not, what is the case?
When Safari was ported to Windows, it came with a port of Apple text renderer. Popular opinion was that the text was blurry and illegible.
People get really, really used to how their system renders text and react very sensitively, and very annoyed indeed, to anything different.