First and foremost they must be operational, so make you test them before heading out to the test.
They must be secure in operation.
They must sweep and clear and area of the windscreen of the opinion of the tester which gives the driver an adequate view of the road ahead and forward of the offside and nearside.
They must operate automatically once switched on.
When switched off the wipers must return to a rest position automatically which is at or beyond the swept area.
When in the rest position the wipers must be capable of being lifted from the windscreen to aid cleaning of the windscreen.