w3schools says that for height:
auto : The browser calculates the height. This is default
But what does “browser calculates the height” mean? Does it mean it just simply sums up the contained elements’ heights? Is this always calculated with the same method for all browser?
It means the browser will make the element’s height sufficient to fit its content.
Nope, it is more complicated than that, but I’m sure that is part of it.
No, there are generally small variances. You should calculate the element’s height in JavaScript if you intend to perform calculations based on it, don’t assume it will have the same height on all browsers.