[css] How to create a drop shadow only on one side of an element?

If your background is solid (or you can reproduce it using CSS), you can use linear gradient that way:

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div {_x000D_
  background-image: linear-gradient(to top, rgba(0, 0, 0, 0) 0%, rgba(0, 0, 0, 0.3) 5px, #fff 5px, #fff 100%)_x000D_
}
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<div>_x000D_
<p>Foobar</p>_x000D_
<p>test</p>_x000D_
</div>
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_x000D_

This will generate a 5px gradient at the bottom of the element, from black at 30% opacity to completely transparent. The rest of the element has white background. Of course, changing the last 2 color stops of the linear gradient, you could make the background completely transparent.