Design criteria can be a help for viewers? (5)
I believe the answer is:
specs
I believe this clue is a double definition.
'design criteria' is the first definition.
(specifications)
'a help for viewers?' is the second definition.
(spectacles)
'can be' acts as a link.
(Other definitions for specs that I've seen before include "Detailed plan" , "you can see right through them" , "Glasses (informal)" , "Instructions" , "what builders are doing?" .)