Responsive font size mixin

This package lets you use the responsive-font-size mixin which automatically calculates the appropriate font size based on the dimensions of the monitor or device.

What is RFS?

RFS (abbreviation for responsive font size) is the name of the algorithm behind the mixin. It's also used by PostCSS RFS and Less RFS.

Advantages

• Font sizes will rescale for every screen or device, this prevents long words from being chopped off the screen on mobile devices.
• The minimum font size (configuration variable) will prevent the font size from becoming too small so readability can be assured.
• Super easy to use, no need to define complex configurations for each font size.
• Font sizes of all text elements will always remain in relation with each other.

Installation

You can use RFS in your project by installing it using a package manager (recommended):

npm:

$ npm install rfs --save

yarn:

$ yarn add rfs

Bower:

$ bower install rfs --save

Copy/paste (not recommended):

You can download the RFS SCSS-file and save it in your scss/ directory. This method is not recommended because you lose the ability to easily and quickly manage and update RFS as a dependency.

Usage

This input (SCSS):

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

Will generate this (CSS):

.title,
.disable-responsive-font-size .title,
.title.disable-responsive-font-size  {
  font-size: 4rem;
}

@media (max-width: 1200px) {
  .title {
    font-size: calc(1.6rem + 3.2vw);
  }
}

You can also use the rfs() alias instead of responsive-font-size().

Configuration

$rfs-minimum-font-size: (in px or rem)
Font sizes which are calculated by RFS will never be lower than this size. However, you can still pass a smaller font size to RFS, but then RFS won't dynamically scale this font size. For example: responsive-font-size(19) will trigger dynamic rescaling, with responsive-font-size(10) it will just stay 10px all the time.
Default value: 1rem

$rfs-font-size-unit: (string)
The font size will be rendered in this unit. Possible units are px and rem.
Default value: rem

$rfs-breakpoint: (in px, em or rem)
Above this breakpoint, the font size will be equal to the font size you passed to the mixin; below the breakpoint, the font size will dynamically scale.
Default value: 1200px

$rfs-breakpoint-unit: (string)
The width of $rfs-breakpoint will be rendered in this unit. Possible units are px, em and rem.
Default value: px

$rfs-factor: (number)
This value determines the strength of font size resizing. The higher $rfs-factor, the less difference there is between font sizes on small screens. The lower $rfs-factor, the less influence RFS has, which results in bigger font sizes for small screens. $rfs-factor must me greater than 1, setting it to 1 will disable dynamic rescaling.
Default value: 5

$rfs-two-dimensional (Boolean)
Enabling the two dimensional media queries will determine the font size based on the smallest side of the screen with vmin. This prevents the font size from changing if the device toggles between portrait and landscape mode.
Default value: false

$rfs-generate-disable-classes (Boolean)
When the the disable classes are generated you can add the .disable-responsive-font-size class to an element to disable responsive font sizes for the element and its child elements. If you don't use this, it's better to set this to false to prevent the generation of unused css. This doesn't apply on font-sizes which are inherited from parents.
Default value: true

!important

By setting a second parameter to true, !important is added after the font-size value.

.label {
  @include responsive-font-size(2.5rem, true);
}

CSS:

.label,
.disable-responsive-font-size .label,
.label.disable-responsive-font-size {
  font-size: 2.5rem !important;
}

@media (max-width: 1200px) {
  .label {
    font-size: calc(1.3rem + 1.6vw) !important;
  }
}

But this generates a lot of css?

True. But with gzip or other compression enabled, the difference in file size is barely noticeable due the high amount of repetitive strings. If you really want to minimize the amount of generated css, setting $rfs-generate-disable-classes to false can make a difference (and lowers the global specificity).

Known issues

Safari doesn't recalculate the value of vw in a calc()-function for font-sizes in iframes if the min-width, max-width or width is not set in vw after the iframe is resized (edge case, but this is the case for Codepen demo's). Adding this line will solve this (dirty fix):

_::-webkit-full-page-media, _:future, :root * {min-width: 0vw;}

Sass syntax

.sass file available in sass/_rfs.sass. Mixin can be used like this:

.title
  +responsive-font-size(36)

Best practices

• Remember to set RFS on your font size of your html or body, otherwise some text may not dynamically rescale (if $rfs-minimum-font-size is decreased). Note that setting RFS on html can influence the value of rem.
• Always set your line-heights relative (in em or unitless).
• More tips and tricks with examples can be found on this article.

Demos

• Simple Codepen Demo
• RFS in bootstrap demo