Here is a paragraph with no background. Thus, the paragraph background is transparent, and shows through the background of the parent (DIV) element.

Here is a paragraph with a background color. Since color is opaque, the background of the underlying DIV is now obscured.

1. Here is an ordered list
2. This list has no background color, so that the background is transparent.
3. Here is a third item.
4. And now a fourth.

P after OL -- moved to overlap

P after P after OL -- moved to overlap