Maximizing canvas space

I have noticed in Chrome and Firefox that attempting to use all of the browser window for webgl by setting the canvas size to (window.innerWidth, window.innerHeight) results in part of the canvas being hidden by browser slider bars. The largest space I can consistently use is (window.innerWidth - 5, window.innerHeight-5)

I sometimes get browser slider bars even with this settings although the slider bars have always disappeared after a browser refresh.