When it comes to determining the "best practice" in this scenario, there are several factors to consider.
Firstly, JSON is primarily a notation language, so as long as the content provided for the template property is a valid string, it can be considered both valid and a recommended approach.
Secondly, the approach to handling the template may vary based on its size. For small HTML templates, wrapping it inside a JSON object might make sense in certain situations.
However, if you are looking to render an entire page using the template, it is advisable to simply provide a link to the template in the property and then retrieve the template using that link. This helps keep your JSON structure clean, easy to read, and significantly simplifies the debugging process.