I'm using Cleanslate to reset all the CSS attributes on my widget container and its children, and I'm trying to add
boder-box isn't supposed to subtract margin.
The width and height properties (and min/max properties) includes content, padding and border, but not the margin
So it seems like your example is working as expected as you are setting a margin:
margin: 12px 20px !important;
that is added to your
Also see this article about box-sizing.