The conditional comment syntax you're using is a IE-only feature (discontinued in IE10) so you can't use that to detect anything else.
With JavaScript however, you can do something like the following;
if (navigator.userAgent.indexOf("iPad") > -1) {
// add classname "iPad" to the <body>
document.body.classList.add("iPad");
}
Then you can prefix any CSS rules with .iPad
to only apply to the iPad.
.iPad .myClass {
/* iPad-specific css */
}
Alternatively, you could use the same test to include an ipad-specific css file:
if (navigator.userAgent.indexOf("iPad") > -1) {
var ipadCss = document.createElement("link");
ipadCss.type = "text/css";
ipadCss.rel = "stylesheet";
ipadCss.href = "/path/to/ipad.css";
document.head.appendChild(ipadCss);
}