It seems like it would be annoying to write this directly, like how do you make a word bold or italic? How do you make a class of hyperlink that's a different color (e.g. internal links vs. external)? HTML is OK but not fun to write directly -- I'm not sure that KDL is an improvement. You would still want something like Markdown.
On the other hand it could be better for tables, which are awkward in HTML. But I'd like to see how that works.
In summary:
- XML/HTML: better when you have more text than data. Good for docs, awkward for tables.
- JSON: better when you have more data than text. Good for web API request and response, awkward for docs (e.g. do you want to write markdown or HTML inside double quotes?). OK for config files but awkward because it's a strict format.