The BBC have an open source library for building apps for TV based on HTML, CSS and JavaScript called TAL: TV Application Layer. It's designed to be deployed across HTML-based TV devices.
http://fmtvp.github.io/tal/getting-started/introducing-tal.h...
Apple's tvOS is likely to have much richer functionality since it's hosted on Apple's own hardware. However, an open standard for TV apps would be a good thing.
The BBCs TAL was released as open source in March 2013. But I don't know if it's gained wider traction in the industry.
You know, without namespaces, you take out the eXtensibility-bit.
SGML, actually. "Just markup" can entail other things, like TeX, for example.
Rather than expecting developers to use an open-spec-compliant generic toolkit, they've just built modules on top of it to both make development easier and more importantly maintain a consistent UX.
You could just have pre-defined CSS classes and do much of the same thing, but it will not be as elegant nor as controlled. Taking over the HTML and JS interpreters just allows them greater control while respecting the role each component plays (markup, interactivity, styling, etc).
The runtime has JavaScript support, it does not have a brand new interpreter.
The runtime has CSS support, it does not have a brand new interpreter.
The runtime has HTML/XML syntax. Why would they write a brand new interpreter?
Mappping XML to a small subset of UI objects is so simple you don't really need to pull in an entire browser runtime for it.
It's significantly more limited and more specific to the task than HTML. HTML comes with a lot of baggage that makes it unsuitable for a really specific use case.
XML does not only look a lot like SGML, it is a proper subset of SGML:
XML is an application profile or restricted form of SGML, the Standard Generalized Markup Language [ISO 8879]. By construction, XML documents are conforming SGML documents.
I don't know if I agree with the tradeoffs they've made here, but those are at least two reasons to create their own custom schema here.
Just imagine all of the garish designs that would pop up.
It's better to restrict what is allowed in a custom markup language so that apple has better control over how it is rendered.
Is there a reason (legal or otherwise) that they can't just use HTML?