<!DOCTYPE html>
<script>
    window.onload = function() {
        var ctx = document.getElementById('c').getContext('2d');
        ctx.fillStyle = 'blue'; // this triggers the lazy creation of the image buffer
    }
</script>
<style>
    canvas { 
        border: solid red;
        background: green;
    }
</style>
<canvas id="c" width="100" height="100"></canvas>