You should see a red square. Inside that square should be a plus-sign, which is centered on the square. If you do not see this, then your browser is incompatible.
If you click the plus-sign, the plus sign should disappear. If you click the red square, nothing should happen. If you click this text, the plus sign should reappear (if it was hidden by clicking it).
This is the basic level of interactivity expected.