I want to add a shadow to the bottom and top of two sections which feature two parallax images. See screenshot below. What would be the best way to add this shadow, in terms of rendering speed, responsiveness, etc.? Thanks for your help!
The "fastest" method is using a top and bottom border if you need a solid line. The next fastest is box shadow as @Ant pointed out.
Note: You may not see the shadow on the bottom unless you set a higher z-index. This is because the section underneath is "higher up" in the document order so it will overlap the shadow.