Comment by cendyne

"Pixel perfect" is about attention to detail and consistency. Margins, padding, or the combination of these inside other containers will stick out when they're not consistent.

Here's an example that I personally encountered: what if you have a <h1>Text</h1> and it has a certain left margin. Then another heading except it has a nested button component (which internally comes with some padding). Then the "Text" in both aren't aligned from section to section and it is jarring.