What does 100% mean in CSS?

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When using percentage values in CSS like this…

.element {
  margin-top: 40%;
}

…what does that % value mean here? What is it a percentage of? There’ve been so many times when I’ll be using percentages and something weird happens. I typically shrug, change the value to something else and move on with my day.

But Amelia Wattenberger says no! in this remarkable deep dive into how percentages work in CSS and all the peculiar things we need to know about them. And as is par for the course at this point, any post by Amelia has a ton of wonderful demos that perfectly describe how the CSS in any given example works. And this post is no different.

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