This tests that a top layer element's containing block is in the initial containing block and that it is unaffected by ancestor elements with overflow or opacity. Dialog elements are used to demonstrate these properties. A dialog opened with showModal() is in the top layer; one opened with dialog.show() is not.

This modal dialog should be onscreen with a width of 90% of the page. It is the child of an narrow element positioned off screen, but the containing block of a top layer element is the initial containing block, so its position and percent lengths are relative to that.
This modal dialog should be unaffected by its ancestor with overflow. It should not be clipped.
This non-modal dialog should be affected by its ancestor with overflow. It should be clipped.
This modal dialog should be unaffected by its ancestor with opacity.
This non-modal dialog should be affected by its ancestor with opacity.