RFS

RFS (abbreviation for responsive font size) is an algorithm which automatically calculates the appropriate font size based on the dimensions of the monitor or device. It's available in 5 languages:

• SCSS
• Sass
• PostCSS
• Less
• Stylus

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

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

npm:

$ npm install rfs --save

yarn:

$ yarn add rfs

Bower:

$ 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.

Usage

SCSS

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

Sass

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

PostCSS

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

Less

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

Stylus

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

Generated css

.title {
  font-size: 4rem;
}

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

Configuration

Minimum font size _{^{(unit in px or rem)}}

SCSS, Sass & Stylus: $rfs-minimum-font-size
Less: @rfs-minimum-font-size
PostCSS: minimumFontSize

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 minimum font size, no fluid font rescaling will take place.
Default value: 1rem

Font size unit _{^{(px or rem)}}

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

The output font size will be rendered in this unit.
Default value: rem

Breakpoint _{^{(in px, em or 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

Breakpoint unit _{^{(px, em or rem)}}

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

Factor _^(number)

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, setting it to 1 will disable dynamic rescaling.
Default value: 5

Two dimensional _^(boolean)

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

Class _^(boolean)

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

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

• 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.
• false
  No extra classes are generated.

Default value: false

Safari iframe resize bug fix _^(boolean)

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

!important

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(1.3rem + 1.6vw) !important;
  }
}

Best practices

• Remember to set RFS on your font size of your html or body (especially if the minimum font size is lowered), otherwise some text may not dynamically rescale. 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 here (written when only the SCSS version was made).

Demos

• Simple Codepen Demo
• RFS in bootstrap demo

Creator

Martijn Cuppens

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

Copyright and license

Code released under the MIT license.