What are CSS percentages?

If you’ve used CSS, you’ve probably used percentage values. They look like this:

body {
  width: 90%;
  margin-left: 10%;
  line-height: 160%;
}

But what are they percentages of? It depends entirely on the property! For the width and margin-left properties, it’s a percentage of “the width of the containing block”. For the line-height property, it’s a percentage of “the font size of the element itself”. And so on, with rather idiosyncratic semantics for each CSS property.

At the bottom of this post is a full list of each CSS property that can take a percentage, along with the semantics for that percentage. But let’s look at a few themes.

Many layout properties are defined in terms of the element’s “containing block” (i.e. usually the element’s nearest block-level ancestor). For example, the width and height properties are percentages of the corresponding width and height of the containing block. Generally, horizontal lengths are percentages of the width of the containing block, and vertical lengths are percentages of the height of the containing block.

So what are margin-top, margin-bottom, padding-top, padding-bottom? Surprise: despite being vertical lengths, they’re percentages of the width of the containing block! Here’s an example:

<div id="square-wrapper">
  <div id="square"></div>
</div>

<style>
  #square-wrapper {
    width: 100px;
  }
  #square {
    background-color: yellow;
    padding-bottom: 100%;
  }
</style>

Due to the odd meaning of padding-bottom: 100%, this gives us a square:

Some properties can’t take percentages. For example, border-width can’t take a percentage. I can imagine it being defined in terms of “the width of the containing block”, but it isn’t.

Finally, here’s a full list of how percentages are interpreted. I scraped it from MDN.

Discussion on Hacker News.

Tagged #programming, #css.

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