Tested with dev-python/html5lib-1.1-r1 - after html5lib merging chromium compiled. Before - it asked for html5lib at the begining and failed. USE="X cups custom-cflags gtk4 hangouts official proprietary-codecs pulseaudio suid system-av1 system-harfbuzz system-icu vaapi wayland widevine -debug -kerberos -libcxx -lto -pax-kernel -qt5 -screencast -system-png"
Full build.log and emerge --info please.