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<!DOCTYPE html>
<html lang="en">
<meta charset="utf-8">
<title>Cross-browser kerning-pairs & ligatures</title>
body { font-family: sans-serif; background: #f4f3f3; color: rgba(40, 30, 0, 1); width: 500px; margin: 80px auto; padding: 0px; }
a { color: rgba(15, 10, 0, 0.8); text-decoration: none; border-bottom: 1px solid; padding: 1px 1px 0px; -webkit-transition: background 1s ease; }
a:hover { background: rgba(0, 220, 220, 0.2); }
p, li { line-height: 1.5; padding: 0em 1em 0em 0em; margin: 0em 0em 0.5em; }
code { background: rgba(0, 220, 220, 0.2); padding: 2px 4px; }
small { font-size: 0.9em; }
hr { background: none; border-top: 1px dotted rgba(40, 30, 0, 0.85); margin: 20px 0px; }
h3 { font-size: 1em; margin: 1em 0em 0.25em; }
h3 + p { list-style: none; padding: 0em 0em 0.5em; margin: 0em; }
h2 { font-size: 5em; margin: 0px; position: relative; }
h2.bask { font-family: 'baskerville', times; font-style: italic; font-weight: normal; }
h2:before { content: 'Normal'; font-family: sans-serif; font-style: normal; font-size: 11px; padding: 2px 5px 1px; background: rgba(25, 20, 0, 0.15); color: rgba(25, 20, 0, 0.6); position: absolute; left: -5.4em; top: 3.5em; font-weight: normal; -webkit-border-radius: 4px; -moz-border-radius: 4px; border-radius: 4px; }
The Magic
h2.kern { text-rendering: optimizeLegibility; }
h2.kern:before { content: 'optimizeLegibility'; left: -9.6em; }
<h1>Cross-browser kerning-pairs &&nbsp;ligatures</h1>
<p>Improved handling of kerning pairs and ligatures in modern browsers using the <code>text-rendering: optimizeLegibility;</code> declaration.</p>
<p>The declaration is currently supported by:<br>
<a href="">Safari 5</a>, The&nbsp;<a href="">Webkit&nbsp;Nightlies</a> & <a href="">Chrome</a>.</p>
<p><a href="">Firefox</a> already uses <code>optimizeLegibility</code> by default for text sizes above 20px.</p>
<p><a href="">Paul Irish</a> was so kind as to pass on a few bugs that have surfaced.</p>
<li>In recent versions of Chromium, mixing <code>optimizeLegibility</code> and <code>font-variant: small-caps</code> will <a href="">cause the small-caps to not render</a>.</li>
<li>Irritatingly, it also causes text to <a href="">disappear completely on webOS</a>.</li>
<li><a href="">Joseph Pearson</a> also points out that on slower machines (such as mobile devices) it can negatively impact page load when applied to large blocks of text.</li>
<li>In Safari 5, when using a measurement type of <code>ex</code> on the <code>margin</code>, <code>padding</code>, <code>border-width</code> or <code>outline-width</code> properties causes the browser to crash. See his demo page here: (warning: this will crash safari, obviously) <a href=""></a></li>
<h2 class="kern">AVAST</h2>
<h2 class="kern">LOVELY</h2>
<h2>True Type</h2>
<h2 class="kern">True Type</h2>
<h2 class="kern">Youth</h2>
<h2 class="bask">traffic</h2>
<h2 class="bask kern">traffic</h2>
<h2 class="bask">floret</h2>
<h2 class="bask kern">floret</h2>
<small>Gist: <a href=""></a></small>

A bit offtopic, but: what's the point of writing background color of body in this manner? — background: rgba(30, 10, 0, 0.05) — this is just #F4F3F3 (faster for rengering, I guess) and you actually relying on default background color in particular browser, which can be changed:

Why I'm asking? It's seriously harms rendering in Opera browser. I know, this is problem of Opera, but I'm just curious.

Ry7n commented Jun 29, 2010

This demo text is a bit deceiving. Safari 4/Mac doesn't appear to support it, and Firefox automatically uses ligatures on text 20px or larger.

Nice improvement for those headlines in Chrome, but I think it's not a good idea to turn it on across the board.


kolber commented Jun 29, 2010

@pepelsbey I was playing around with background contrast and found it much easier than trying to work out the hex backwards. I wouldn't recommend doing this on a production site.

@RCanine thanks, I will update the text to reflect this. I have done some basic testing with larger amounts of text and it doesn't seem to actually take a noticeable performance hit.

I've created a Chrome/Chromium extension that automatically turns this CSS property on for every page. It can be found here:

I filed a bug report to Apple yesterday, telling that Safari crashes when I use text-rendering: optimizeLegibility together with either margin, padding, border-width or outline-width measured in ex. It works fine with em, px, in, % etc., though. I’ve put together an example page at


kolber commented Oct 21, 2010

Thanks for the heads up.
I've added the bug to the caveats list.

I've spent nearly a day trying to work out why Android 2.3's Browser (via browserstack) was crashing out completely on a fairly simple page. I looks to be suffering the same ex-units bug as the Safari versions discussed already. Perhaps an old Webkit issue rather than just Safari? It seems resolved in newer Androids and iOS versions, but it's worth noting if you're legacy device testing.

Zeokat commented Mar 9, 2014

Zeokat nice theme, thanks!

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