Use the Gtk reference example and
press the "Show Alpha Features" from the "Tools" menu to display the relevant features used in this example.
The body itself has a transparent background, set through CSS.
SVG Opacity
<div> Opacity
I am text in the div
Note that I (the text) have the same opacity as my div
PNG Opacity
ATVEF Punchthrough

You can see right through the window above. Regardless of what is
beneath it, ATVEF images punch through it all.