Detect Android phone via Javascript / jQuery

Take a look at that : http://davidwalsh.name/detect-android

JavaScript:

var ua = navigator.userAgent.toLowerCase();
var isAndroid = ua.indexOf("android") > -1; //&& ua.indexOf("mobile");
if(isAndroid) {
  // Do something!
  // Redirect to Android-site?
  window.location = 'http://android.davidwalsh.name';
}

PHP:

$ua = strtolower($_SERVER['HTTP_USER_AGENT']);
if(stripos($ua,'android') !== false) { // && stripos($ua,'mobile') !== false) {
  header('Location: http://android.davidwalsh.name');
  exit();
}

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Edit : As pointed out in some comments, this will work in 99% of the cases, but some edge cases are not covered. If you need a much more advanced and bulletproofed solution in JS, you should use platform.js : https://github.com/bestiejs/platform.js

I think Michal's answer is the best, but we can take it a step further and dynamically load an Android CSS as per the original question:

var isAndroid = /(android)/i.test(navigator.userAgent);
if (isAndroid) {
    var css = document.createElement("link");
    css.setAttribute("rel", "stylesheet");
    css.setAttribute("type", "text/css");
    css.setAttribute("href", "/css/android.css");
    document.body.appendChild(css);
}

How about this one-liner?

var isAndroid = /(android)/i.test(navigator.userAgent);

The i modifier is used to perform case-insensitive matching.

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Technique taken from Cordova AdMob test project: https://github.com/floatinghotpot/cordova-admob-pro/wiki/00.-How-To-Use-with-PhoneGap-Build