The issue comes from something called "margin collapsing". It's simple: 2 adjoining margins collapse to the highest of the two (I say two, but it could be more).
In your case, '#mydivs' margin-top - 80px - is touching the 'header's margin-top - 0px. They're adjoining - there's no element between them, nor padding, nor border.
The margins collapse, therefore, to the highest of the two (80px), and it is then applied on the highest of the elements in the parent-child hierarchy - that's the header in this case.
One solution to this problem is to put something between the margins; either of some padding, or a border on the header works fine.
header {
border-top: 0.1em solid rgba(0,0,0,0);
}
A second solution (my preferred one), is to make the parent element create a new block formatting context. That way, its margins simply won't collapse with that of its child.
How do you create a block formatting context?
There are four possible ways.
- by floating it.
- "position absoluting it".
- adding one of these displays: “table-cell”, “table-caption”, or “inline-block".
- adding an overflow other than visible.
To prevent the margins from collapsing you could do any of these 4. I usually go for number 4) - set overflow to auto, as it's only side affect... well it's improbably likely to become a problem.
header {
overflow: auto;
}
That's basically it for parent-child margin collapsing. There's also margin collapsing between siblings and the rule is pretty much the same: 2 adjoining margins collapse to the highest of the two. It's the solutions that are not.
Here's a great explanation of margin-collapsing - http://www.howtocreate.co.uk/tutorials/css/margincollapsing
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