Edit in JSFiddle

<h1>Animated checkboxes using iconfonts</h1>

<!-- A list of checkboxes -->
<ul>
	<li>
		<input type="checkbox" name="one" id="one" />
		<label for="one">Create checkbox</label>
	</li>
	<li>
		<input type="checkbox" name="two" id="two" />
		<label for="two">Assign label</label>
	</li>
	<li>
		<input type="checkbox" name="three" id="three" />
		<label for="three">Import iconfont</label>
	</li>
	<li>
		<input type="checkbox" name="four" id="four" />
		<label for="four">Iconify label pseudo elements</label>
	</li>
	<li>
		<input type="checkbox" name="five" id="five" />
		<label for="five">Animate icon widths</label>
	</li>
	<li>
		<input type="checkbox" name="six" id="six" />
		<label for="six">Color the icons</label>
	</li>
</ul>
@charset "utf-8";
/* CSS Document */

/*Montserrat for the text*/
@import url(http://fonts.googleapis.com/css?family=Montserrat); 

/*basic reset*/
* {margin: 0; padding: 0;}

body {
	font-family: montserrat;
	padding-top: 10px;
	color: #333;
}

h1 {
	font-size: 16px;
	padding: 15px;
	text-align: center;
}

ul {
	width: 290px;
	margin: 0 auto;
}
ul li {
	list-style-type: none;
	padding: 10px;
}

/*Adding custom checkbox icons*/
label {
	position: relative;
	padding-left: 30px;
	font-size: 14px;
	cursor: pointer;
}
label:before, label:after {
	font-family: FontAwesome;
	font-size: 21px;
	/*absolutely positioned*/
	position: absolute; top: 0; left: 0;
}
label:before {
	content: '\f096'; /*unchecked*/
}
label:after {
	content: '\f046'; /*checked*/
	/*checked icon will be hidden by default by using 0 max-width and overflow hidden*/
	max-width: 0;
	overflow: hidden;
	opacity: 0.5;
	/*CSS3 transitions for animated effect*/
	transition: all 0.35s;
}

/*hiding the original checkboxes*/
input[type="checkbox"] {
	display: none;
}
/*when the user checks the checkbox the checked icon will animate in*/
input[type="checkbox"]:checked + label:after {
	max-width: 25px; /*an arbitratry number more than the icon's width*/
	opacity: 1; /*for fade in effect*/
}

/*adding some colors for fun*/
#one+label:before, #one+label:after {color: hsl(0, 45%, 40%);}
#two+label:before, #two+label:after {color: hsl(60, 45%, 40%);}
#three+label:before, #three+label:after {color: hsl(120, 45%, 40%);}
#four+label:before, #four+label:after {color: hsl(180, 45%, 40%);}
#five+label:before, #five+label:after {color: hsl(240, 45%, 40%);}
#six+label:before, #six+label:after {color: hsl(300, 45%, 40%);}