CSS is a powerful, intricate language that gives the web beauty and life. But it also can make you want to throw your computer out of a four story window and never touch the front-end again.

Most frustration with CSS comes from a lack of understanding, which makes sense since it is often taught as an afterthought of web development. If CSS was treated like a language rather than a coat of paint, I think we would appreciate its value more.

All of the CSS we write eventually boils down to pixels displayed on a screen for our users to see and engage with. But the term pixel can be misleading when we are talking about the values in our stylesheets. It stands to reason that a "pixel" would be the smallest visible quantity that can be displayed on a screen. But in reality, a pixel is just... really small. (1/96 of an inch apparently, but who's counting)

Here's what the spec says:

The reference pixel is the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm's length. For a nominal arm's length of 28 inches, the visual angle is therefore about 0.0213 degrees.

I was not enlightened by that explanation, so I took some bigfoot-esque photos of different physical screens displaying a single yellow CSS pixel.

That's an older LG Monitor, an iPhone 8+, and a 2018 15" MacBook pro.

It looks like our 1 css pixel equated to: 1 screen px for the LG, maybe around 6 or 8 for the iPhone, and 4 screen px for the MacBook. That's just conjecture though, this is the important part:

So what?

It's important to understand how our tools work, and when it is best to reach for them. px isn't the only fish in the sea.

The CSS px value should be used when you need an absolute unit of measurement for an element that will not fluctuate in size on your page. (Or it will change very little)

There are other units of measurement in CSS that can offer flexibility and responsiveness in ways that px fails to.

div {
  height: 10em;
}

div {
  height: 10rem;
}

div {
  height: 10vh;
}

div {
  height: 10%;
}

em: Font size of the parent element

rem: Font size of the root element

vh: 1% of the viewport's height

%: Usually a percentage of the nearest "sized" parent element (There are a lot of caveats to this one)

In my personal opinion, if you need a unit of CSS measurement that offers the most flexibility and control, rem wins hands down. It is more predictable than em, and makes responsiveness a breeze when changing layouts or sizes for different screens. I won't get into the weeds here, it was very thoroughly explained in this wonderful article.

Hopefully you have a little extra nugget of knowledge when it comes to CSS units of measurement, cheers! 🍻

Pixel art by Clint Bellanger