Do they have: 1) strong IDE support 2) rich control libraries ?

XAML doesn't necessarily have strong IDE support, either. Although integrated into VS, it really is no comparison to the "REPL" of HTML/CSS + your favorite text editor and a browser.

Not sure I understand your use of REPL - in Visual Studio you edit XAML in the right pane, and the change is reflected in the left pane as you type. That's more efficient than editing HTML or CSS in a text editor, saving, switching to browser, refreshing page. Can you explain?

That's entirely dependent on your workflow isn't it? I can set up a workspace within dev tools and edit HTML/CSS/JS on the fly and have the changes reflected on disk and on screen without ever losing sight of my work. Add in the networking tools (timelines, profiles etc..) and I have full featured 'IDE' that I can also browse hackernews with.

The XAML editor in Visual Studio was broken for years. Blend, once you learn how to use it, is a much nicer tool.

In any case, unlike HTML you really don't want to be writing XAML by hand.

I write all my XAML by hand. Blend is a bit too cavalier for my liking, especially when building for Windows Phone.

1) There are ST plugins if that counts 2) No, though I'm sure you could make do with snippets for now, and web components in the future.