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.