This unstable branch of webkitgtk only works with bleeding-edge versions of 
gtk4 (>= 4.11.2). It will not compile with the latest stable gtk4.

Stable and unstable webkitgtk libraries (soversions 4.0 and 6.0) can
coexist on the same system, but /usr/bin/WebKitWebDriver can only be
owned by one of them. This port avoids the conflict by appending "-6.0"
to the bin filename. You should place a symlink (or a wrapper script) at
the expected location /usr/bin/WebKitWebDriver once you decide which
version of webkitgtk you want to prioritize.