User blog comment:Argonash11/Why can't Quarians have see-through visors?/@comment-2036282-20100713132638

Since Tali was the only Quarian seen in ME1, I think Bioware decided to keep intriguing gamers by rather than uncovering their faces. Instead of this, Bioware simply added much more facts about Quarian culture and so on. We can only hope they reveal Tali's face in ME3.

Maybe the quarian visor may be covered with a material which protects quarian's face from sun radiation or something, but also darkens the glass. But it's just my speculation.

And the mystery, of course. One of the major reasons of Tali's popularity, I suppose.