[KeyStruck]

Use the CSS3 specification's @media (media queries) attribute. What it generally does is detect "WHAT DEVICE", and then use a different style sheet based on what device you're using. You can set if statements making it look like so:

[xhflhkeuhwguu34]

Complexity, and routing. Sometimes you will not be able to determine if the user is mobile or not based off of their user agent.

It's also a pain to be redirected to another subdomain and lose your progress.

It's also a LOT easier to make a site responsive then to build a 2nd one (typically).