This article is out of date and the code linked should not be used for anything right now. Please, refer to the more recent How to Work on WebKit
To work on the Haiku WebKit port one needs to take some time to get the right environment set up. At the moment the Haiku WebKit port can only be developed on Linux and cross compiled with the GCC4 compiler. So the first step is to follow my tutorial on building Haiku on Ubuntu. On step 5 of the above, please be sure to build the GCC4 cross compiler, not the GCC2 version. If you have already built Haiku with GCC2 on Linux, I recommend renaming your current "generated" directory in the Haiku tree to "generated_gcc2" and then continue with step 5 from the above, setting up the GCC4 cross compiler. Switching between compilers can then be done by just renaming the generated directory, since it contains all compiled build tools and necessary files for each compiler.