I am project lead for FHIR, a widely endorsed specification developed by HL7 for the exchange of healthcare information (http://hl7.org/fhir). We are trying to converge the healthcare community on using CommonMark for rich text. One of the challenges we have with this is that CommonMark does not support underline. I’ve read Feature Request: underline text, and I understand the theoretical reasons why underlining has problems, but replacing the practice of underlining with some other text decoration is not a simple thing - people have been underlining text for a long time, and existing healthcare documents make heavy use of underling. In a few cases, the text will explicitly referenced ‘underlined text’, or existing clinical protocols will do so, but new software based on markdown instead of RTF cannot support the clinical protocol.
I’m not sure how to progress this. If CommonMark doesn’t and won’t support underline, we could define our own extension, but that has very obvious downsides, and we don’t want to do that. Alternatively, GFM could, because we’re actually using GFM, for strikethrough (critical for healthcare documents). Otherwise… maybe any consensus to use commonmark might be impossible (? not sure, but clinical users feel extremely strongly about text fidelity).
An aside: text color is also a problem, but since most systems have not supported color, and because of color blindness, color is used and depended on a lot less.