Normal behavior:

Selected text should be green.
Selected text should have a green background (forecolor undefined).
Selected text should be green (background undefined).

Propagation behavior:

Does ::selection inherit in your browser ? (it should)
[Does your browser correctly apply ::selection to generated contend (it should)]
Does image selection overlay (here:) honor the background property ?
Does your browser support tables correctly ?

Combination behavior:

Does your browser support selection opacity (it shouldn't)
Does your browser support selection color opacity (it should; you can hover for test)
Does your browser support selection color opacity (it should; you can hover for test)