UPDATE Support for this feature has been dropped. Please seek other options
Original Post:
You may want to look at scoped styles; see http://css-tricks.com/saving-the-day-with-scoped-css/.
The basic idea is
<div>
<style scoped>
@import "scoped.css";
</style>
</div>
However, you are on the bleeding edge here in terms of browser support. See http://caniuse.com/style-scoped.
One alternative would be to use an iframe.