<!DOCTYPE html>
<html>
<link rel="stylesheet" type="text/css" href="/fonts/ahem.css" />
<style>
div {
    margin-left: 3px;
}
div::before {
    width: 100px;
    height: 100px;
    margin-left: 2px;
    content: 'foo';
    color: rgba(0, 0, 0, 0);
}
canvas{
    margin-left: 2px;
    display: block;
    position: relative;
    top: -1em;
}
</style>
<body style="font: 10px/1 Ahem;">
    <div><canvas width="30px" height="10px"></canvas></div>
<script>
document.addEventListener('DOMContentLoaded', function() {
    var canvas = document.querySelector('canvas');
    var context = canvas.getContext("2d");
    context.fillStyle = 'green';
    context.fillRect(0, 0, 30, 10);
});
</script>
</body>
</html>
