When using the CSS JIT, the task of matching a selector is split in two: first compiling, then testing. A JIT compiler takes the selector, does all the complicated computations when compiling, and generates a tiny binary blob corresponding to the input selector: a compiled selector. When it is time to find if an element that matches the selector, WebKit can just invoke the compiled selector.
In the most recent versions of the JIT, the compilation phase is within one order of magnitude of a single execution of SelectorChecker. Given that even small pages have dozen of selectors and hundreds of elements, it becomes easy to reclaim the time taken by the compiler.
There is ongoing work to support everything SelectorChecker can handle. Currently, some pseudo types are not supported by the JIT compiler and WebKit fall backs to the old code. The missing pieces are being added little by little.
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