Inside OnCreate, add:
webview.getSettings().setSupportZoom(true);
webview.getSettings().setBuiltInZoomControls(true);
webview.getSettings().setDisplayZoomControls(false);
Inside the html document, add:
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=2, user-scalable=yes">
</head>
</html>
Inside javascript, omit:
//event.preventDefault ? event.preventDefault() : (event.returnValue = false);