[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.