Hackability

To make rapid progress possible, we try to keep the code relatively easy to understand, even though web technologies are often complex. We try to use straightforward algorithms and data structures when possible, we try to write clear, maintainable code, and we continue to improve names and code structure to aid understanding. When tricky “rocket science” code is truly needed to solve some problem, we try to keep it bottled up behind clean interfaces. In addition, we make heavy use of automated regression tests as a safety net, to allow aggressive changes with less risk of regressions.

(via WebKit Project Goals)