Fragments Layout on a template

[aviranbergic]

We would like to understand how you layout your fragments on a template and manage the margins and alignments.
meaning if we have

<fragment id='header' slot='header'></fragment>
<fragment id='promotions' slot='content'></fragment>
<fragment id='footer' slot='footer'></fragment>

what is your approach to layout the fragments on the page correctly?
where and how do you manage the CSS?

each fragment internally has its surrounding margins and positioning or is it done by the template?