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On Variables, Calculations, and New Units

Published 15 December, 2012 Updated 7 August, 2019

Disclaimer: Alright, don't get too excited. This stuff isn't backwards-compatible at all. However, there are fallback solutions and it's pretty darn cool that it's actually being spec'd.

Web designers were begging for variables and calculations in CSS well before LESS and Sass came along However, Sass and LESS output can sometimes bloat your CSS and, while it's pretty fun to play with, it's a pain to have to recompile your CSS after every change. Fortunately, the W3C has come in like the proverbial Mary Poppins and to give them a spit shine, making everything all fresh and new again.

Variables

Mmmmmmmmm, variables. We've used them in almost every language but CSS… Until now. Admittedly, CSS variables are quite a long way off. They're still in a W3C Level 1 module so you won't be able to play with them for a while unless you want to get into the Webkit nightly builds. Still, it's fun to think about all of the amazing things we're going to be doing with them in a year…

Here's what variables might look like in CSS3. Keep in mind that since it's still Level 1, it's subject to change. To declare a variable, you'll create it inside an element. All of the examples are declaring them in the :root{} using the var prefix like this:

:root {
 var-mainColor: #0c0;
}

You can then call the variables later on in the code by using the var() syntax:

p {
 background-color: var(mainColor);
}

My main use for this in CSS has been dealing with branding colors. I can set all of the branding colors in the top of the stylesheet and use the variables throughout code, like this:

:root {
 var-primaryColor: #0d56a6;
 var-secondaryColor: #4186d3;
 var-tertiaryColor: #689ad3;
}
header {
 background-color: var(primaryColor);
}
header > nav {
 background-color: var(secondaryColor);
}
article {
 background-color: var(tertiaryColor);
}
button {
 background-image:
   linear-gradient(
     bottom,
     var(primaryColor) 0,
     var(secondaryColor) 50%,
     var(tertiaryColor) 100%
   );
}

From this point on if I want to change the color scheme I can just change the colors in my :root element and they'll change across the entire website! I do hope, however, that before the spec is released we get the ability to create full property set variables that work like SASS mixins. The ability to do something like this would be stupendous:

var gradient (color1, color2, color3) {
 background-image:
   linear-gradient(
     bottom,
     var(color1) 0,
     var(color2) 50%,
     var(color3) 100%
   );
}
article {
 gradient(#0d56a6, #4186d3, #689ad3);
 border: 1px solid #000;
}

W3C Module

Calculations

Calculations are in a W3C Level 3 module, meaning they're pretty close to making it into the CSS3 full spec. If all goes well, we'll be able to consider them totally safe to use come next June. However, we can already play with them in the latest version of Chrome, Firefox, Opera, and Safari, as well as IE9/10.

So why are calculations so exciting? How about this horizontal centering:

body {
 margin-left: calc(960px / -2);
 width: 960px;
}

MDN Page | W3C Module

Units

There are some really snazzy new units coming with CSS3 but the ones I'm most excited about are rem, vw, vh, vmin, and vmax.

rem

We're all aware of the em which we're encouraged to use for the sake of magnified page viewing. However, I've avoided them for quite sometime because they're a major pain in the ass to try to calculate. I don't like having to figure out what my em value is based on it's parent's calculated value which is based on it's parent's calculated value which is based on it's parent's calculated value which is… It just sucks. The rem, however, is here to save the day! The rem is similar to the em except that it's always relative to the em value of the root of the page. Take this code:

:root {
 font-size: 20px;
}
.box1 {
 font-size: 2em;
}
.box1 > .box2 {
 font-size: 2em;
}
.box3 {
 font-size: 3rem;
}
.box3 > .box4 {
 font-size: 1rem;
}

In this case, .box2 will have a font-size of 80px (2em of 20px = 40px, then 2em of 2em of 20px = 80px) but .box4 will have a font-size of 20px (.box3 would be 3rem of 20px = 60px, .box4 if 1rem of 20px = 20px). It's actually even more complicated than that, but this is the general way to figure everything up. rems are much easier to use then ems, hands down.

vw and vh

Even more awesome are the units that calculate based on the size of the viewport! vw and vh are percentages of the viewport width and height, respectively. Each vw/vh is 1% of the viewport size, so think about this:

nav {
 width: 100vw;
}
nav a {
 min-width: 100px;
 width: 20vw;
}

With this you can create a navbar with 5 elements that is always the same size as the viewport! The min-width also makes sure that the links will never be too small so you can stack them or turn them into a vertical menu for super tiny screens.

vmin and vmax

These guys are always relative to the larger or smaller size between the vh and vw. In other words, if the width of the viewport is smaller than the height (maybe its a smartphone in portrait mode) then the vmin will be relative to the width and the vmax will be relative to the height.

W3C Module