How to change the “html” element's CSS

Question

html {
    width:100%;
}

How to change the CSS of the html tag dynamically on clicking the button using JavaScript? I mean I want to make

Answer 1

There's probably a simpler way, but

htmlTags = document.getElementsByTagName("html")
for(var i=0; i < htmlTags.length; i++) {
    htmlTags[i].style.overflowY = "hidden";
}

Hope I remembered everything right.