If this doesn’t use a compiler, the ”when” feature looks like it will require 'unsafe-eval' and potentially 'unsafe-inline' to be enabled in content security policy, which will disable browser XSS protections. Not optimal for an easy-to-use component system intended for display of remote data.
Edit: I don’t see a CSP on the dogfooded homepage so I would assume this is an issue.
XMLUI does not execute JavaScript with "eval" or anything that would not comply with CSP. It runs interpreted JavaScript asynchronously in a sandbox and does not allow use of "eval" (and another dozen low-level functions, such as setTimeout).
Edit: I don’t see a CSP on the dogfooded homepage so I would assume this is an issue.