Calculate a page's base em

script.js

function calculate_base_em() {
	var d = document.createElement('div');
	var base_height;

	d.style.opacity = 0;
	d.innerHTML = 'E';

	document.body.appendChild(d);

	base_height = d.offsetHeight;
	d.parentElement.removeChild(d);

	return base_height;
}