IntraWeb has built in support for Bootstrap, JQueryUI and more.
IntraWeb implements these libraries as loadables. The libraries needed for these controls are loaded on demand so as to keep bandwidth and load time to a minimum. You can easily implement your own custom controls as a loadable as well.
Loadables are marked with a namespaces to identify them. To use a Bootstrap button, we use BS.Button, where BS. is the namespace for Bootstrap. To use Bootstrap all you need to do is reference a Boostrap control and IntraWeb will do the rest.
You can use existing Bootstrap layouts by using a template, or you can implement Bootstrap layouts using IWML using BS.Row and BS.Col.
Now we have a Bootstrap button instead of a the built in IntraWeb button. All we had to do was add the namespace for Bootstrap (BS.) and now we have a different button. The BS.Button is upward compatible with the built in Button, as the BS.Button inherits from Button.