This script creates a small red box at the bottom left with the viewport dimensions.

display-viewport-script.js

/*
  Script to display the viewport size when working on responsive stuff.
  Adpted to vanilla JS by: Taylor Hunt - https://codepen.io/tigt/
*/
var el = document.createElement("output");
document.body.append(el);
Object.assign(el.style, {
    position: "fixed",
    bottom: 0,
    left: 0,
    background: "red",
    color: "white",
    padding: "5px",
    fontSize: "11px",
    opacity: 0.7
});
function updateOutput() {
    var html = document.documentElement;
    el.value = html.clientWidth + " × " + html.clientHeight;
}
window.addEventListener("resize", updateOutput);
updateOutput();