[xkjyeah]

It probably allowed their engineers to code the design bits in an easier language (imagine, HTML vs C++!), allowing them to iterate and customise the design more quickly.

It's probably in the same vein as how phone apps that need a new feature quickly may decide to simply start a WebView.

[jeroenhd]

Microsoft hid their web views well in many places. The special decorations rendered by Windows explorer were largely done through HTML, but they managed to make them feel native.

I think their efforts to make their own browser-based controls feel like native controls was what proved thst Microsoft did actually want the web control for convenience.

Sadly, these days that native feel is hard to come by. UI toolkits are now super complex and the "solution" seems to be to port the entire UI toolkit to a Javascript runtime, then not maintain that for a while and let it deviate from the native standard, and launch a new Javascript runtime to make up for it. Updating anything in the OS has become easy, so you don't need to design give years ahead. In two years, you can just roll out an update and change the look and feel of the built-in applications.