When qtgui:5 is compiled without the "jpeg" USE flag, qtwebkit:5 ends up being compiled without jpeg support as well (as documented in its Tools/qmake/mkspecs/features/features.prf file). For an HTML rendering engine, that makes it rather crippled. So it would make sense for it to either depend on qtgui[jpeg] directly, or have a jpeg USE flag that depends on qtgui[jpeg].
dev-qt/qtwebkit-5.212.0_pre20180120 does not depend on qtgui[jpeg] anymore.