Pure form of single authoring is considered impractical except for the lowest common denominator, which fails to meet the content providers needs with respect to end-users. Not going to be solved by one language. But this not required in all applications.
Application model has to address state, interaction and transactions.
Not all applications will use all of these. Promising Solutions - Embedded One approach is to embed information about presentation and interaction into the content. Constraint-based mechanisms can simplify the task of the author by pushing the burden back on the implementers.