Jim Nielsen’s Blog
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Width and Height in CSS

In his video “Secret Mechanisms of CSS”, Josh explains (among other things) how width and height work in CSS. I loved his explanation so much, I am going to re-write it here for my own benefit. Hopefully the next time I have to explain it — to someone else, or to myself in my head — I’ll be able to do it as clearly as Josh.

The secret is in this summation: to determine width you look up the tree, to determine height you look down the tree.

For example, take this code:

    <div>My content</div>
      div {
        width: 100%;
        height: 100%;

width looks up the tree and fills the space made available by its parent. In this case it will look all the way up to the <body> tag which looks to the <html> tag which, by default, has the width of the document. So the <div> does too.

Conversely, height looks at its children. It asks, “How big are all the elements inside of me?” and then automatically fills that height.

The key insight here is: height: 100% means “as tall as all the things inside of me”, not “as tall as all the things I am inside of”.

That is why width: 100% does what a lot of people naturally expect: it goes the full width of the screen. Whereas height: 100% doesn’t do what people often expect and go the full height of the screen.

Again, width looks outward while height looks inward.

To be honest, I struggled with this idea for many years. Even when I knew width: 100% would give me what I wanted and height: 100% wouldn’t, I never fully understood why. Josh’s explanation is a great way to think about it. As he notes in his talk:

That's the funny thing about CSS. You can have a mental model that is 95% correct and go through your career with this slight gap [in understanding] that you never notice until one day it doesn’t line up and then there goes your afternoon.

So true.