If you want a data format which expresses some specific subset of HTML, well, do that then. Again, validate on output that the text you're showing is within the defined subset and escape everything else. Eg, "<strong> is passed verbatim to the browser but any other < character is replaced with <".
This technique still works fine. You just need to also do the work of defining what your data format looks like, and how it should be parsed and displayed in a web browser.
