After discovering the Surf minimalistic web browser, I was just curious on how difficutl would it be to use the XEmbed protocol to wrap it into another application. It is far easier than I thought initially by using the gtk.Socket class which does implement the protocol in a convenient way, for example using the following Python code:

import gtk, sys

socket = gtk.Socket()
window = gtk.Window()
window.set_title(u"Embedded widget")
window.add(socket)

# Embed *after* inserting the socket in a window!
socket.add_id(int(sys.argv[1]))
window.show_all()
gtk.main()

Save this in a file (e.g. embed.py) and now you can run Surf the following way:

# Running surf with -e will print the X window ID
surf -e -u http://google.com &
python embed.py <window-id>

(Thanks go to Claudio for pointing out the gtk.Socket/gtk.Plug classes)