How to create a positioning system in css + sass?

Question

css

I really don't like css, but still I have to work with it. My question is - as the helpers say, a child can be set to 100% sizes only if all of its parents also have 100% sizes. And it annoys me to think that a parent, who is not so little recruited, needs to set these very sizes. I write in sass. Can you advise me something? How do you deal with it? Or is it normal for you?
And I will ask in advance - what to do if the parent should not have one hundred percent sizes?
UPD: 0.0.1
I'm sorry I forgot to say - I also use bootstrap3, so everything I said earlier is really using it. And developing the topic, I would be glad to hear what solution the css + sass gurus chose for themselves in the matter of positioning?
UPD: 0.0.2

Answer 1

a child can only be set to 100% sizes only if all of its parents also have 100% sizes.

some nonsense. and if the parent has 50% width - that's it, won't it work? :)
Another question is that in order to correctly determine the width as a percentage, the parent must have a position, and usually relative. but this is not the case in all cases.

Answer 2

Any element by default has a size of auto, which is usually equivalent to 100%. So we don't fight