The function of the windshield on a snowmobile (or snow machine, as some prefer to call them) has little to do with safety: it's there to shield the operator from the wind (thus, in this case, the name windshield describes exactly what it does) and to deflect snow from him or her. Operating as snowmobiles do in sometimes brutal winter conditions, where the temperature, already low, is made even worse by the wind chill caused by the speed of the machine, safety requirements are completely different from normal road or leisure vehicles. They can be summed up easily: ability to withstand the conditions; sufficient strength to deflect small objects; correct positioning to deflect snow blown by the wind or from the machine directly ahead; and sturdy mountings.
Windshields vary from the perfunctory — on racing machines where air resistance is to abolished as much as possible — to the comfortable, which the operator can hide behind and which will also keep the snow off his/her feet. Interchangeability from one manufacturer to another is not something that should be counted on — that's to say, there are windshields for each specific machine. And the quality can vary from purely functional to show-offy.
There do not seem to be any legal standards from snowmobile windshields. Because of this, it is advisable to buy only from reputable dealers. Acrylic plastic windshields are preferable to glass as they stand up better to the conditions. Repair of the windshields is possible but for other than scratches is probably not worth the trouble. Do-it-yourself installation or replacement is usually easy: just a matter of screws.