Got this building on FreeBSD for those interested:

1. Install the GTK stack:

  pkg install gtk4 libadwaita webkit2-gtk_60 pkgconf

2. FreeBSD ships WebKitGTK 2.46.6 and the project turns on the v2_50 feature of the webkit6 crate, so switch that to v2_46:

  sed -i '' 's/v2_50/v2_46/' $(git grep -l v2_50 -- '*Cargo.toml')

After that, cargo build completes against 2.46.