Just to add a different element/perspective to what jx said eloquently, with CSS with every ‘div’ you’ve kind of got another ‘box’ inside a ‘box’, rather not quite unlike those classic ‘Matryoshka dolls’.
‘absolute’ positioning totally does help for ‘children’ of the ‘parent’, but sometimes it can even still feel a little confusing to figure out exactly what it is all ‘relative to’.
Sometimes I find it useful in my thinking to just conceptually ‘color’ the parent, draw out its box, or print it out to help in my thinking ‘okay, I know I am inside this higher level div, thus its position is relative’, but relative to ‘what’ ?
To be honest, I still think the ‘relative canvas’ model, used in a lot of ‘straight up’ computer graphics would make a lot more sense to me. But this sort of ‘relative design modality’ is not all that foreign to traditional graphic designers.
I’m not a GD by trade, but personally I found this book pretty enlightening as to the design audience much of CSS3/HTML5 is sort of ‘speaking to’.
It is also kind of a ‘classic’ in its field in its own right.
Hope this is of some help…