<div style="background-color: #666">
<div style="margin: 20px">Some Content - no border</div>
</div>
However, if I add a border, background color is applied to the whole content including child element's margin:
<div style="background-color: #666; border: 1px solid">
<div style="margin: 20px">Some Content - has border</div>
</div>
What's the reason for this behavior? What's the specification explains this?