onFontReady, a 653 byte function that fires a callback after a font has loaded.

onFontReady.js

function onFontReady() {
	var
	fonts = [].slice.call(arguments),
	callback = fonts.pop(),
	element = document.documentElement.appendChild(document.createElement('x'));

	element.setAttribute('style', 'clip:rect(0 0 0 0);font:9px _,serif;position:absolute');

	element.innerHTML = ' ';

	var lastOffset = element.offsetWidth;

	element.innerHTML += '<style>@font-face{font-family:_;src:url(data:font/ttf;base64,AAEAAAAKAIAAAwAgT1MvMiW9IsAAAAEoAAAAYGNtYXAAPwEJAAABkAAAADxnbHlmFiY2OAAAAdQAAAAYaGVhZPmD8i4AAACsAAAANmhoZWEC8P8JAAAA5AAAACRobXR4AAIAAAAAAYgAAAAIbG9jYQAMAAwAAAHMAAAABm1heHAABAAEAAABCAAAACBuYW1lAAYAAAAAAewAAAAGcG9zdAADAAAAAAH0AAAAIAABAAAAAQAANDQG4V8PPPUAAAPoAAAAAM0H14AAAAAAzWfZ0AAAAAAAAQABAAAAAAACAAAAAAAAAAEAAALu/wYAAAABAAAAAAABAAAAAAAAAAAAAAAAAAAAAAACAAEAAAACAAQAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAgAAAZAABQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAAAAAAAAAAAAAAAAICAgIAAAACAAoALu/wYAAAL1APoAAAABAAAAAAAAAAAAAAAAAAAAAQAAAAEAAAAAAAEAAwABAAAADAAEADAAAAAIAAgAAgAAACAALQCg//8AAAAgAC0AoP///+H/1P9hAAEAAAAAAAAAAAAAAAwADAAAAAEAAAAAAAEAAQADAAA1MxUjAQEBAQAAAAAAAAAGAAAAAwAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA==)}</style>';

	function init() {
		if (element.offsetWidth !== lastOffset) {
			element.style.fontFamily = fonts+',_';

			load();
		} else setTimeout(init);
	}

	function load() {
		if (element.offsetWidth) {
			document.documentElement.removeChild(element);

			callback.call(this, fonts);
		} else setTimeout(load);
	}

	init();
}