WebKit GTK (headful) requires libffi.so.6 which is not available on Ubuntu 20.04 (list of available packages).

The closest match is libffi7, and my workaround is to symlink it as libffi.so.6:

$ find /usr/lib -name "libffi.so*"
/usr/lib/x86_64-linux-gnu/libffi.so.7

$ sudo ln -s /usr/lib/x86_64-linux-gnu/libffi.so.7 /usr/lib/x86_64-linux-gnu/libffi.so.6

Filing this for others that might run into it, and to verify if this is the best way.