RFS (simply the abbreviation for Responsive Font Size) is an font size engine which automatically calculates the appropriate font size based on the dimensions of the browser viewport. It's available in one of your favourite preprocessors or postprocessor: Sass, Less, Stylus or PostCSS.

• Font sizes will rescale for every screen or device, this prevents long words from being chopped off the viewport on small devices
• RFS will prevent the font size from rescaling too small so readability can be assured
• Super easy to use, just use the font-size mixin (or responsive-font-size property for PostCSS) instead of the font-size property
• The font sizes of all text elements will always remain in relation with each other

RFS can be installed using a package manager (recommended):

• npm: npm install rfs
• yarn: yarn add rfs
• bower (deprecated): bower install rfs --save

Copy/paste (not recommended):

The source files can also be downloaded manually and used in a project. This method is not recommended because you lose the ability to easily and quickly manage and update RFS as a dependency.

In the following examples, this folder structure is assumed (you will probably just use one pre/postprocessor):

project/
├── postccss/
│   └── main.css
├── less/
│   └── main.less
├── node_modules/
│   └── rfs
│        └── ...
├── sass/
│   └── main.sass
├── scss/
│   └── main.scss
└── stylus/
    └── main.styl

// .scss syntax:
// scss/main.scss

@import "../node_modules/rfs/sass/rfs";

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

// .sass syntax:
// scss/main.scss

@import "../node_modules/rfs/sass/rfs";

.title
  +responsive-font-size(4rem) // OR +responsive-font-size(64px) OR +rfs(64)

// postcss/main.css

.title {
  responsive-font-size: 4rem; // OR responsive-font-size: 64px; OR rfs: 64;
}

// Handle postcss afterwards (see examples folder for PostCSS example)

// less/main.less

@import "../node_modules/rfs/less/rfs";

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

// stylus/main.styl

@import "../node_modules/rfs/stylus/rfs";

.title
  responsive-font-size(4rem) // OR responsive-font-size(64px) OR rfs(64)

.title {
  font-size: 4rem;
}

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

If you wonder how the font sizes are rescaled, wonder no more and stare at this graph which might clarify things a bit. The font sizes used on the graph are in px, but in reality RFS renders them in rem by default:

RFS works out of the box without any configuration tweaks, but if you feel the urge to go loco and fine tune the way font sizes are rescaled, you can:

• SCSS, Sass & Stylus: $rfs-base-font-size
• Less: @rfs-base-font-size
• PostCSS: baseFontSize

The option will prevent the font size from becoming too small on smaller screens. If the font size which is passed to RFS is smaller than this base font size, no fluid font rescaling will take place.

Default value: 1.25rem

• SCSS, Sass & Stylus: $rfs-font-size-unit
• Less: @rfs-font-size-unit
• PostCSS: fontSizeUnit

The output font size will be rendered in this unit. Setting it in px will disable the ability for users to change the the font size in their browser.

Default value: rem

• SCSS, Sass & Stylus: $rfs-breakpoint
• Less: @rfs-breakpoint
• PostCSS: breakpoint

Above this breakpoint, the font size will be equal to the font size you passed to RFS; below the breakpoint, the font size will dynamically scale.

Default value: 1200px

• SCSS, Sass & Stylus: $rfs-breakpoint-unit
• Less: @rfs-breakpoint-unit
• PostCSS: breakpointUnit

The width of the max width in the media query will be rendered in this unit.

Default value: px

• SCSS, Sass & Stylus: $rfs-factor
• Less: @rfs-factor
• PostCSS: factor

This value determines the strength of font size resizing. The higher the factor, the less difference there is between font sizes on small screens. The lower the factor, the less influence RFS has, which results in bigger font sizes for small screens. The factor must me greater than 1.

Default value: 10

• SCSS, Sass & Stylus: $rfs-two-dimensional
• Less: @rfs-two-dimensional
• PostCSS: twoDimensional

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

• SCSS, Sass & Stylus: $rfs-class
• Less: @rfs-class
• PostCSS: class

RFS can be enabled or disabled with a class. There are 3 options:

• false No extra classes are generated.
• disable 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.
• enable RFS is disabled by default in this case. The .enable-responsive-font-size class can be added to an element to enable responsive font sizes for the element and its child elements.

Default value: false

• SCSS, Sass & Stylus: $rfs-safari-iframe-resize-bug-fix
• Less: @rfs-safari-iframe-resize-bug-fix
• PostCSS: safariIframeResizeBugFix

Safari doesn't resize its font size in an iframe if the iframe is resized. To fix this min-width: 0vw can be added and that's what happens if this option is enabled. See #14.

Default value: false

By setting a second parameter to true, !important is added after the font-size value. (Example is in scss)

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

CSS:

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

@media (max-width: 1200px) {
  .label {
    font-size: calc(1.375rem + 1.5vw) !important;
  }
}

• Don't set RFS on the html element, because this influences the value of rem and could lead to unexpected results.
• Always set your line-heights relative (in em or unitless) to prevent interline issues.

• Simple Codepen Demo
• RFS in bootstrap demo

Martijn Cuppens

• https://twitter.com/Martijn_Cuppens
• https://github.com/MartijnCuppens

Code released under the MIT license.