It should be possible to make tests at [[Special:ExpandTemplates]] without reloading the whole page after every small change in the code.
There is already an API for that, except for the option "Suppress <nowiki> tags in result", it seems:
https://en.wikipedia.org/wiki/Special:ApiSandbox#action=expandtemplates&title=Test&text=%7B%7BU%7CExample%7D%7D
Version: 1.23.0
Severity: enhancement