Until recently, the code for calculating intersections between lines and exclusion shapes was using its own coordinate system. This was in addition to the two main coordinate systems used in WebKit, physical and logical (sometimes called abstract) from the CSS Writing Modes Specification. After some mulling, Hans decided to do some additional up-front processing and use logical coordinates rather than our own home-brewed variety. As it turns out, two coordinate systems are plenty. Read more over at his blog.