What does 100% mean in CSS?

Amelia Wattenberger:

One of the CSS units I use most is the wonderful % — so handy for positioning elements on the page.

Unfortunately, the rules aren’t exactly straightforward. One question I’m always asking myself is: “Percent of what?”

Hopefully this guide can help clear things up.

As per usual: It Depends™

Amelia being Amelia here she sprinkled plenty over beautiful and interactive examples for you to play with across the page. Very nice!

What does 100% mean in CSS? →

💁‍♂️ The fact that percentages (sometimes) are calculated against the width of an element enabled us to create aspect-ratio boxes in CSS. That method is being supplanted nowadays, as CSS Sizing Level 4 specifies a aspect-ratio CSS property.