The second link isn't applicable to Apple's WebKit ports — it's entirely built via Xcode. https://github.com/WebKit/WebKit/blob/39386f4547897c89c510d0eeeecddc5

The second link isn't applicable to Apple's WebKit ports — it's entirely built via Xcode. https://github.com/WebKit/WebKit/blob/39386f4547897c89c510d0eeeecddc5efb5d6d13/Source/WebCore/Configurations/WebCore.xcconfig#L29-L34 https://github.com/WebKit/WebKit/blob/39386f4547897c89c510d0... defines USE_JPEGXL only for macOS < 14 (which aren't actually supported any more!). All the in-tree JPEG XL support, e.g., https://github.com/WebKit/WebKit/blob/39386f4547897c89c510d0eeeecddc5efb5d6d13/Source/WebCore/platform/image-decoders/jpegxl/JxlDecoderPtr.h https://github.com/WebKit/WebKit/blob/39386f4547897c89c510d0... is behind a "USE(JPEGXL)" ifdef — so none of that is compiled in. Instead, it's using what Apple ships at a system level, in Image I/O. https://github.com/WebKit/WebKit/blob/39386f4547897c89c510d0eeeecddc5efb5d6d13/Source/WTF/wtf/PlatformHave.h#L1315-L1321 https://github.com/WebKit/WebKit/blob/39386f4547897c89c510d0... defines HAVE_JPEGXL for recent versions of Apple's OSes. https://github.com/WebKit/WebKit/commit/932073284e4c73ce9884d0c6e15fe86fe99685c5 https://github.com/WebKit/WebKit/commit/932073284e4c73ce9884... is the commit which added this — there's really not much there, because it's just setting the define and adding it to the allowlist of image types. And yeah, currently I believe this is libjxl — or a fork thereof — hence the inclusion of libjxl in the Acknowledgements.rtf file on macOS.