When WordPress 3.5 beta 1 was released it came with, “Twenty Twelve”. While looking at style.css, I noticed the use of “REM” to set font sizes and margins. I had NO idea what REM was. In fact, I just started using EM’s in other child themes. Immediately, I turned to Google and started researching.
I thought REM was a band?
By definition, the REM unit is: “Equal to the computed value of ‘font-size’ on the root element. When specified on the ‘font-size’ property of the root element, the ‘rem’ units refer to the property’s initial value.“
tl;dr The REM is one of the newest font-size standards for CSS3. It stands for “Root EM”. You base your calculation on the default font size of the HTML element – NOT the parent-like EM. It’s basically “EM v2.0”.
So what?
The problem with Pixels is, they absolutely do-not-scale in Internet Explorer. Furthermore, with the onset of Responsive Web Design, having fonts that scale (in relation to the screen width) has become paramount. Percentages (%) and EMs are better, but they’re tricky and compound. Still not an answer. A real solution?
Use the REM
First, you need to set a default “root” font-size variable:
html {
font-size: 62.5%
}Why 62.5% instead of 100%? Simplicity.
Our default font is now 10px, which makes math easier. Now, 1.0rem = 10px. This becomes our $rembase.
Calculate other font sizes by dividing your desired size by the $rembase (in pixels). Let’s say, you want your <h1> tags to be 26px:
26 ÷ 10 = 2.6remor
32 ÷ 10 = 3.2rem
48 ÷ 10 = 4.8remand so on…Let’s take a look at a sample Stylesheet:
/* $rembase = 10px */
html {
font-size: 62.5%;
}
body {
font-family: 'Helvetica Neue', Arial, sans-serif;
font-size: 1.0rem; /* 10 ÷ $rembase */
}
h1,
h2 {
font-size: 2.6rem; /* 26 ÷ $rembase */
}
h3,
h4 {
font-size: 1.8rem; /* 18 ÷ $rembase */
}That looks simple enough, right? Just move the decimal. Now your fonts will scale perfectly during a browser re-size (if using responsive design), or if a user were to zoom in or out.
But what about Line Height?
Line heights have always given me headaches, that was until Chris Pearson released his Golden Ratio Typography Calculator. Now, figuring out line heights (among other settings) is a snap.
- Input your desired font size (16px)
- Specify how wide your content box is (mine is 740px)
- Click “Set my type!”
The Golden Ratio Typography Calculator spits out optimized typography values:
The calculator tells us our default line-height should be 26px. This variable is the $line-height-base.
We’re going to be using “Unitless line heights” as explained by Eric Meyer, so we can avoid unexpected results. What’s so awesome about the unitless line-height? You only have to specify it once in the <body> tag. Now, ALL other line-height(s) are relative to the parent font size. That’s too easy! (Of course, you can still specify your own to maintain complete control.)
To calculate, you divide the $line-height-base by the font size for that particular element (in pixels).
26 ÷ 16 = 1.625
How would that look in our sample style sheet?
/*
$fontbase = 16
$line-height-base = 26
*/
html {
font-size: 62.5%;
}
body {
font-family: "Helvetica Neue", Arial, sans-serif;
font-size: 1.6rem;
line-height: 1.625; /* $line-height-base ÷ $fontbase */
}
h1,
h2 {
font-size: 2.6rem;
line-height: 1; /* $line-height-base ÷ 26 */
}
h3,
h4 {
font-size: 1.8rem;
line-height: 1.444 /* $line-height-base ÷ 18 */
}Note: We’ll only use three decimal places to the right since most browsers only calculate that far.
What about margins?
Yes, you can even use REM to set your margins. Margins, or “vertical spacing” is calculated using either 24px or 48px to maintain vertical rhythm. Let’s divide 24px by our $rembase:
24 ÷ 10 = 2.4remHere’s our sample stylesheet:
/*
$rembase = 10
$line-height-base = 26
*/
.some-div {
margin: 2.4rem 0; /* 24 ÷ $rembase */
}
.another-div {
padding: 4.8rem; /* 48 ÷ $rembase */
margin-bottom: 2.4rem; /* 24 ÷ $rembase */
}I’m convinced, now tell me about browser support.
At the time of writing: REMs are supported in Firefox, Chrome, Safari, Opera, and yes, even IE 9 & 10. It’s also supported in all mobile browsers except for Opera Mini.
Check out this list: http://caniuse.com/rem
What about fallbacks?
As is the case with most CSS3 wizardry, REM support in IE 6, 7, and 8 is lacking and will require a PX fallback. By declaring REMs after PXs in the CSS this example will degrade gracefully to the PX:
html {
font-size: 62.5;
}
body {
font-family: "Helvetica Neue", Arial, sans-serif;
font-size: 16px;
font-size: 1.6rem;
line-height: 1.625;
}
h1,
h2 {
font-size: 26px;
font-size: 2.6rem;
}
h3,
h4 {
font-size: 18px;
font-size: 1.8rem;
line-height: 1.444;
}
.some-div {
margin: 24px 0;
margin: 2.4rem 0;
}The purpose of this post was not to stand on a soapbox and preach but educate you on the advantages of using REMs to enhance the typography on your site. Your comments are welcome below.
PX to REM Sass Mixin
// Create REM values with PX fall back
//
// Generate a REM with PX fallback from
// $baseFontSize. Enter the desired size based
// on pixels in numerical form. Supports shorthand.
//
// Forked from: http://codepen.io/thejameskyle/pen/JmBjc
//
// @author Greg Rickaby
// @since 1.0
//
// Usage: @include rem($property, $values);
// Example Usage:
// @include rem(font-size, 16px);
// @include rem(margin, 0 24px 0 12px);
//
// Outputs:
// font-size: 16px;
// font-size: 1.6rem;
// margin: 0 24px 0 12px;
// margin: 0 2.4rem 0 1.2rem;
// ----------------------------------
$baseFontSize: 10; // Based on HTML reset html { font-size: 62.5%; }
@function parseInt($n) {
@return $n / ($n * 0 + 1);
}
@mixin rem($property, $values) {
$px : ();
$rem: ();
$root: $baseFontSize;
@each $value in $values {
@if $value == 0 or $value == auto {
$px : append($px , $value);
$rem: append($rem, $value);
}
@else if type-of($value) == number {
$unit: unit($value);
$val: parseInt($value);
@if $unit == "px" {
$px : append($px, $value);
$rem: append($rem, ($val / $root + rem));
}
@if $unit == "rem" {
$px : append($px, ($val * $root + px));
$rem: append($rem, $value);
}
}
@else {
$px : append($px, $value);
$rem: append($rem, $value);
}
}
@if $px == $rem {
#{$property}: $px;
} @else {
#{$property}: $px;
#{$property}: $rem;
}
}
@function rem($value) {
$root: $baseFontSize;
$val: parseInt($value);
$return: ();
@if unit($value) == "px" {
$return: append($return, ($val / $root + rem));
} @else {
$return: append($return, ($val * $root + px));
}
@return $return;
}
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