Detecting Windows with JavaScript and CSS 6/29/15

By Chris Johnson

Recently, I wanted Microsoft Windows users to see different CSS styles from users on other platforms. 1 My solution was to use JavaScript to add the class windows to the root html element if the user was on Windows and not-windows if they weren’t. With that class in place, I could write platform specific CSS.

The JavaScript is simple, all it has to do is check the handy navigator.platform property for the substring win, and add the class to the html element depending on the result.

Here’s the working script I added to my HTML document <head> above my CSS styles 2:

// add html class if we're on windows
var htmlEl = document.getElementsByTagName("html")[0];
var className = "not-windows";
if( typeof navigator.platform !== 'undefined' && navigator.platform.match(/win/i) ){
  className = "windows";
if (htmlEl.classList){
else {
  htmlEl.className += ' ' + className;

With that JavaScript in place I could write CSS code like this:

/* Use Comic Sans on Windows and Helvetica Neue everywhere else */
.windows body {
  font-family: 'Comic Sans MS', sans-serif;
.not-windows body {
  font-family: 'Helvetica Neue', Arial;

  1. Windows was garbling a custom web font that Mac OS X, iOS, and Linux had no trouble displaying. It didn’t matter what browser you were using. 

  2. If the JavaScript was loaded after the CSS, visitors might see odd styling flashes when the windows or not-windows class is added.