I've been able to recreate the error. The solution is to clear the browser's cache, after each saved modification.
Other elements may or may not be supported. In any case we are not planning any updates to the feature's current implementation.In fact we've stopped using it in our projects, as the current implementation is not adequate (both in scope of html elements supported and in performance when handling complex pages), and implementing it in full is not warranted by known project requirements.
However, we do think that the ability of designing the display using standard HTML editing tools is a viable requirement, and we are considering other implementation options - possibly importing and recreating the display in a model based on the relatively new Display/HTML Tag template.
Sorry if this causes you any inconvenience.
For best results, use the Firefox browser..