function createCanvas(container, width, height, opt_classname) {
        var canvas = document.createElement('canvas');
        canvas.className = opt_classname ? Runner.classes.CANVAS + ' ' +
            opt_classname : Runner.classes.CANVAS;
        canvas.width = width;
        canvas.height = height;
        container.appendChild(canvas);

        return canvas;
    }
Create canvas element.
@param {HTMLElement} container Element to append canvas to.
@param {number} width
@param {number} height
@param {string} opt_classname
@return {HTMLCanvasElement}