Using a Mixin to Take the Math out of Responsive Font Sizes

Responsive Font Size (RFS) is an engine that automatically calculates and updates the font-size property on elements based on the dimensions of the browser viewport.

If you’re thinking that sounds familiar, that’s because there is a slew of tools out there that offer various approaches for fluid typography. In fact, Chris compiled a bunch of those a little while back. Check that out because it’s always good to know what’s out there and what fits best for a particular task.

RFS is different in that it makes writing code for fluid type feel a lot like writing native CSS (or, more accurately, like writing with a preprocessor) directly in the stylesheets you’re already working in — only without having to wrangle and manage a bunch of media queries. It’s even compatible with Sass, Less, Stylus and PostCSS, so it plugs into just about any stack.

Just how integrated is it? Well, let’s compare a snippet for fluid typography that uses the calc() function…

html {
  font-size: 16px;
}

@media screen and (min-width: 320px) {
  html {
    font-size: calc(16px + 6 * ((100vw - 320px) / 680));
  }
}

@media screen and (min-width: 1200px) {
  html {
    font-size: 22px;
  }
}

…with a similar example of how it can be done with RFS in Sass:

.title {
  @include font-size(4rem);
}

Which compiles to:

.title {
  font-size: 4rem;
}

@media (max-width: 1200px) {
  .title {
    font-size: calc(1.525rem + 3.3vw);
  }
}

Curious how that works? Let’s check it out and then go into how to set it up for a project.

The magic behind automatic re-scaling

Here’s a graph to get a better understanding of how RFS re-scales font sizes:

Every color represents a font size that gets passed to the font-size() mixin provided by RFS. The y-axis of the graph represents the font size (in px) and the x-axis represents the width of the viewport (again, in px).

Let’s focus on the green line, which is generated by applying a mixin to an element:

.title {
  @include font-size(40);
}

In this case, a font size of 40px is passed into the mixin. That value serves as the maximum font size of the element and reaches that size when the viewport is 1200px or wider, at which point it stays at that size.

Conversely, the font size will bottom out at 20px, never going below that mark.

Everything else? Well, that’s where the font size value is automatically calculated, using a function behind the scenes to determine the number according to the current width of the viewport.

RFS is also a little opinionated in that it limits itself to font sizes that are 20px and above. The reasoning is that smaller text (e.g. normal body text) normally does not need to flex as much and is a lot easier to manage than larger pieces of content, like titles and such. That’s very much in line with FitText, which also prefers being used on large text (even though it will not stop you from doing it).

If you’re the type of person who likes to look under the hood, the mixin for each preprocessor is available to view in the RFS GitHub repo. For example, here’s a direct link to the SCSS version. It’s a lot of math!

Note that every font size is generated in a combination of rem and vw units, but they are mapped to px in the graph to make it easier to understand. In other words, it really takes all the mathwork out of the mix.

Everything is configurable

Seriously. Every. Single. Thing.

For example, you may have wondered why the font size capped out at viewports 1200px and wider in the previous example. That can be changed, as well as a ton of other things, including:

• Base font size: The lowest font size value.
• Font size unit: The type of unit to use in the output value (px or em).
• Breakpoint: The maximum width of the viewport where the font size of the element reaches its maximum value.
• Breakpoint unit: The unit used for the media query that the mixin generates (px, em or rem).
• Factor: This serves as a sorta volume control that informs the mixin how aggressive it should be in calculating font sizes from the maximum viewport width all the way down.
• Rem value: This defines the value of 1rem in pixel (px) units.
• Two dimensional: A feature that sniffs out the smallest side of a viewport and uses it to calculate the font size value. This comes in handy when, say, you’d like to keep the font from getting smaller when a device is rotated from a portrait orientation to landscape.
• Class: Provides class names that can be added to an element in the HTML to either enable or disable fluid sizing.

So, yeah. A lot of options and flexibility here. The important thing to know is that all of these options are variables that can be defined in your stylesheets.

All this said, the default settings are pretty safe to use, and they will prevent a lot of longer words truncating from the viewport. This is especially true for some languages — like German or Dutch — that contain a lot of compound words.

Using RFS in a project

Let’s dive straight into to the code. It would be exhaustive to look at the code for each preprocessor, so I’ll be explaining everything in the .scss syntax. But if you prefer something else, you can check out the examples in other languages in the GitHub repo in the Usage section.

First and foremost, RFS needs to be installed on the project. It’s available in npm and Yarn:

## npm
npm install rfs

## Yarn
yarn add rfs

## Bower is available, but has been deprecated
bower install rfs --save

Then, gotta make sure the mixin is imported with the rest of the styles, wherever you do your imports for other partials:

@import "~rfs/scss";

Now, we can start cooking with the mixin!

.title {
  color: #333;
  @include font-size(64px);
}

.subtitle {
  color: #666;
  @include font-size(48px);
}

.paragraph {
  @include font-size(16px);
}

I passed values in px, but rem units are also supported. If a value without a unit is passed, px is used by default. The font sizes are always rendered in rem (in combination with vw) to make sure the font sizes also increase when the default font size is increased in the browser (this is a feature often used by visually impaired people).

The output is:

.title {
  color: #333;
  font-size: 4rem;
}

@media (max-width: 1200px) {
  .title {
    font-size: calc(1.525rem + 3.3vw);
  }
}

.subtitle {
  color: #666;
  font-size: 3rem;
}

@media (max-width: 1200px) {
  .subtitle {
    font-size: calc(1.425rem + 2.1vw);
  }
}

.paragraph {
  font-size: 1rem;
}

Notice that the mixin is font-size(), but RFS will also let you use it in two other ways:

.title {
  @include font-size(4rem);
  // or
  @include responsive-font-size(64px);
  // or
  @include rfs(64);
}

RFS is baked right into Bootstrap

Here’s a little story for you.

One day, I had this incredibly impulsive idea to put RFS into Bootstrap. I actually did not use Bootstrap at that time, but believed it was a feature Bootstrap could definitely use. I made a pull request and waited a couple months to see what would happen.

In the meantime, I was getting more and more intrigued by Bootstrap and version 4 had just been released. Slowly but surely, I got more involved in contributing to the project and a whole new world opened for me when I discovered the community behind it. It was during hacktoberfest (oh yes, I got my t-shirt) in October 2018 that I got asked to join the Bootstrap team by mdo.

I believe contributing to open source projects is such a fun and rewarding thing. Andrés Galante has a great post on the topic if you’re interested in becoming a contributor.

Since then, RFS has become a project of the Bootstrap team, and on February 11th this year, we launched Bootstrap 4.3 which includes RFS right out of the box. It’s currently disabled by default, but can easily be switched on by setting the Sass variable $enable-responsive-font-sizes: true.

But make no mistake: RFS can still be used on its own. Just cool that it’s baked right into a widely used framework.

Oh yeah, let’s talk browser support

Support is pretty darn good! In fact, RFS will work anywhere that supports media queries and viewport units. RFS will set a font size for Legacy browsers, like Internet Explorer 8, but the fluidity won’t be there. In other words, should be safe for production!

What’s next for RFS

The next major version of Bootstrap is version 5 and we’re planning to enable RFS by default. We don’t have any plans to change the way it works for now. More than likely, the $enable-responsive-font-sizes variable will simply be set to true and that’s it.

In the future, I hope I can make use of the min() function because it would generate less CSS and make things a lot less complex. Browsers don’t seem to support this function all too well just yet, but if you’re interested in this feature, you can follow the progress in this GitHub issue.

Anything else? No, but I can leave you with a little song and dance: Na na na na, na na na na, hey hey hey goodbye!