We've hit a few items related to differences with how MSVC's preprocessor behaves. https://bugs.webkit.org/show_bug.cgi?id=198604 https://bugs.webkit.org/show_bug.cgi?id=180709 The experimental preprocessor seems to behave similar to what the clang/gcc preprocessor does.
MSVC Preprocessor Progress towards Conformance | C++ Team Blog https://devblogs.microsoft.com/cppblog/msvc-preprocessor-progress-towards-conformance/