HTML is a first-class citizen in Opa: it is a predefined data type with special support that allows it to be entered using its usual syntax.
If you are a web framework, then support HTML as a first-class citizen is useful and advantageous. In this case, Opa achieves that by inventing a new programming language. Is there other ways? Right off the top of my head, I would think of Tcl might be an candidate.