Remember that the apprearance of all of these background elements can be modified in fairly profound ways by adjusting the Advanced Properties like _alpha, _rotation, _xcale & _yScale, and so on.

When you have set _alpha to make the element semi-transparent, you can then overlap several semi-transparent background elements to get interesting visual effects.

You can use a Px control to provide an image as part of your background if you like. This image can also be modified by the Advanced Properties, to rotate it, make it transparent, enlarge it, and so on.

Some (but not all) background elements can be placed so that only part of them shows in the Stage area, so using combinations of these techniques gives you enormous capabilities that are just not available in HTML.