First you need to install these packages:

sudo pacman -S git autoconf automake gtk3 webkitgtk
git clone --depth 1 https://github.com/emacs-mirror/emacs.git (or git protocol if you like)
cd emacs
./autogen.sh all
./configure --with-xwidgets --with-x --with-x-toolkit=gtk3 --with-modules
make
cd lisp
make autoloads
make
make
make

Until you got everything ok.

Then you can just:

cd src
./emacs

And then, M-x webkit-browse-url RET: xwidget-webkit

Also, test the modules feature using syohex/emacs-qrencode: qrencode

I’m using ssh and X11 Forward to show Emacs in Mac OS X! Cool!