Proper form depends on the activity...

Proper form for martial arts is not the same as proper form for powerlifting. And the proper form for testing is not the same as proper form for training...

We as human beings are dogmatic and want clear cut one size fits all answers and solutions. We also want one method to work for all situations.

Example - Your knees should never go past your toes. Says who? Try going up stairs with out letting your knees go past your toes. Watch an athlete and show me how their knees never go past their toes. We do it every day - but here are the caveats to letting the knees go past the toes.
#1 - You must display proper hip movement and stability. (ie...your glute medius and hips must do what they are supposed to do - if not the knee pays the price)
#2 - Your arch should be maintained. (if you pronate {lose your arch} everything up the chain must compensate - and your knee pays the price)
#3 - If you are going to add resistance to the "knees over the toes" movement it should be light or not at all.

We love rules and answers. Just do _____________ and all will be well - is what we are looking for in our lives.

It just ain't that simple.

Proper form for Martial Arts will entail "tucking the hips under" (think Tai Chi, Qigong etc...) but proper form for a power-lifting back squat will entail sitting back into the hips (sticking the butt out) and maintaining a lumbar curve. Each has it's place depending on the goal.

Proper form for testing the deep squat is with the toes straight ahead. Once you can show me that - Proper form for the weighted squat is with the toes having a slight turn out (15-30 degrees depending on the individual). Toes straight ahead under load will push you against your limits for your hips - not a great idea. Once you show me full range of motion in the testing - you can work with the turn out to provide a little buffer.
Hip mechanics, pull of the hip flexors etc...dictate that this his how we operate under load.

So - are you trying to transfer "proper form" for one activity into a hard and fast "rule" that you are applying to all things - or do you recognize that proper form is dependant on the activity and goal?