I am not sure where the difference between a custom template operator and a custom template function is, for me they are just two names for essentially the same thing.
Without getting an example of what you were trying to achieve, it is hard to say whether there is a better solution, but generally I wouldn't return html from a custom template operator. After all I can add the html in the template.
So post an example and I'll have a look at it.
Claudia
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