That is always tricky. HTML is a presentation language in many ways since the use of tricks or inline CSS is not forbidden. So the problem of interaction with it is similar with the problem of interaction with LaTeX (which however adds on top the macro problem). We are doing research on conservative conversion from LaTeX and probably similar techniques could be used for HTML. However I think would be more meaningful to aim to Markdown or some restricted versions of HTML as common format, since this will is easier to map reasonably well to the TeXmacs document format. I wonder if tools like PanDoc have these bidirectional conversion capabilities themselves. We are planning to have a conservative conversion for Markdown documents (meaning that local edits in TeXmacs only change locally the original Markdown document)