But I've never seen any layout render engine with the same feature set as modern HTML/CSS engines.

HTML has more features about layout. Also styling is more flexible in HTML.

XAML has better rendering (esp. in WPF), better alpha blending (esp. in UWP), much better animations, better vector graphics, and better editor (you can create acceptable layouts using WYSIWYG, not touching raw markup). Also it has templating, data binding, and other features not found in HTML.

Other technologies cited are very limited, not even close to what current web technologies offer.

Because until recently even basic stuff like centering text or grid layouts were an exercise in frustration.

Not to mention the magical incantation dance with CSS transforms to try to force a browser to use hardware acceleration.

Or nonsense like Shadom DOM to be able to define some sort of components, which are only properly supported on Chrome.