Use descendant invalidation set for :indeterminate pseudo class.

On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

PASS getComputedStyle(checkbox, '').backgroundColor is transparent
PASS internals.updateStyleAndReturnAffectedElementCount() is 1
PASS getComputedStyle(checkbox, '').backgroundColor is green
PASS successfullyParsed is true

TEST COMPLETE