This tests that the methods of the EventTarget interface work
correctly.
There are three sub-tests (a rectangle for each). These are:
- Sub-test 1: test adding a listener with addEventListener.
-
Sub-test 2: test adding a listener with addEventListener then
removing it with removeEventListener.
-
Sub-test 3: test adding a listener for an event type with incorrect
casing.
The test has been passed if all three rectangles are blue.