"So it's just HTML? Is there a reason (legal or otherwise) that they can't just use HTML?"
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.
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.
I have no idea how it's actually implemented, but it's quite likely TVML is powered by HTML (and WebKit).
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).
It's not like HTML except for being XML which looks a lot like SGML.
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.
It's not like HTML except for being XML which looks a lot like SGML.
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.
Supporting full HTML would much be more processor- and memory-intensive. It also gives developers more freedom, which can negatively impact the user experience.
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.
I think the heavy reliance on these templates is a good clue. Apple TV's interaction model is a lot about swiping around from tappable element to tappable element. Arbitrary layouts will make this a miserable experience. Apple probably wants to establish a number of layout patterns that they believe will yield a good experience.
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.