So I want my main CSS container to be transparent enough to let a background image show through. That's easy.
But I don't want the content -- text and images -- inside the container to be transparent. So a viewer will see the background image wherever there is white space in the container, but the text and images will completely cover that background image.
Make a small gif (like 5x5 pixels) thats all transparent and then make every other pixel black (it will look like a checkerboard pattern when zoomed really far in). Then assign it to repeat with css and it will seem transparent.