Usually business applications are split in three tiers: user interface layer (UI), business logic layer (BLL) and data access layer (DAL). That split is also known as Model-view-controller architectural pattern.
Ideally each layer should be isolated from the rest. For many good reasons. In any case each layer operates using its own vocabulary (namespace if you wish). For example “Account” in UI means some set of fields (<fieldset> literally). In BLL it is an instance of some class with attributes and methods describing logic. And in DAL it could be just a record in DB with associated triggers and stored procedures.
Code that works on each layer(domain) is serving different tasks. Ideally each such layer may require different programming languages that are optimal for each particular layer. E.g. DB layer as a rule uses TSQL as it better suits DB manipulation needs.
The same thing is about UI. Usually UI tasks are just sequence of rules like “if user clicks here than enable that thing over there”. Such tasks may or may not be handled equally well by universal languages like C++.
Ideally UI layer should have its own language that is better suitable for UI needs. In the same way as DB has its own language (couple of them actually: DB schema definition language and transaction definition language).
After many years of designing various UI and UI toolkits I shall admit that UI automation/programming is not quite OOP task in general. Usually it is better describable in terms of something close to functional programming but with the ability to define classes/objects.
UI is an assembly of sometimes very different UI components/controls. That requires language that is flexible enough to glue them together. That is my vision on TIScript in Sciter.
And of course markup and CSS are also Good Things for the UI. Especially CSS.