Enter "Federated UI Markup Language" (pronounced 'Fweemel').
And it's even based on an existing standard!
The design objective of the User Interface Markup Language (UIML) is to provide a vendor-neutral, canonical representation of any user interface (UI) suitable for mapping to existing languages. UIML provides a highly device-independent method to describe a user interface. UIML factors any user interface description into six orthogonal pieces, answering six questions:
What are the parts comprising the UI?
What is the presentation (look/feel/sound) used for the parts?
What is the content (e.g., text, images, sounds) used in the UI?
What is the behavior of the UI (e.g., when someone clicks or says something)?
What is the mapping of the parts to UI controls in some toolkit (e.g., Java Swing classes or HTML tags)?
What is the API of the business logic that the UI is connected to?
Inevitably, there will be other proposals for federated UI markup languages. I expect we'll see something that will be styled as "composable & modular". Maybe another will be a Interface Metasystem Interoperability ...... no, probably not.