For those that have integrated something like htmx/server side rendered HTML over an API into existing JSON-oriented APIs - what's the cleanest way you've found

For those that have integrated something like htmx/server side rendered HTML over an API into existing JSON-oriented APIs - what's the cleanest way you've found to have both formats exist together? Not that every existing API call would need to include an HTML output, but for many projects I could see there being many endpoints that would also make sense to output HTML and wouldn't deserve an entirely different API. My first thought was to add a format/output parameter to specify the format. In that case of HTML output, you could possibly use the JSON that would normally be rendered and insert it into a jinj2 template for example.