[pjmlp]

> The killer feature of building UI elements in code is that you can build up more complex widgets from simpler ones using composition.

You can do this in XAML since 2006, and JSP since 2004.

[whowouldathunk]

Not sure why you're being downvoted. XAML definitely supports complex widget composition in markup. Since the XML is basically just describing an object graph, it's a more concise way of writing the equivalent imperative code:

    <Button>
        <StackPanel>
            <Image/>
            <TextBlock/>

    ...

    <Button>
        <Canvas>
            <Ellipse/>

although the Dart syntax makes this look similar, albeit a little more verbose, if you squint.

Full disclosure: I work at Microsoft.