Apply a very short gradient to an element, from mostly transparent dark colour to the normal background colour.
Normally you'd use an inner shadow to make an element like a field look as if it's inset in a page, but if for some reason you can't achieve the effect you want that way, you can use a very short gradient instead. Most gradients change colour from the top of an element to the bottom, but this gradient gets all of its colour changing done very early.
If the gradient is too long, you might not get a very realistic "inset" effect.
