CSS is truly full of surprises.
Let's say you have a simple HTML:
<div class="with-background">
    <div class="content with-margin"></div>
</div>

Now, to ensure that margin of the internal element won't "fall out" of the background, you might add padding: 0.1px to the wrapper. But, here's a fun fact: adding 0.01px still works fine, but 0.001px does not.

For my browser the cut-off seems to be somewhere around .008333332371px