@alisha-- I'm not an expert but I do some markup language for my personal site. As far as I know, all of those are universal languages for markup. SGML stands for "Standard Generalized Markup Language," HyTime is something like "Hypermedia Time Language" and DSSSL is "Document Style Semantics and Specification Language."
I think all of these came out because there were too many markup languages and one markup language could only be used for one program and not for others. It became necessary to have a universal language for markup that could be used for most if not all computer programs. But as you can see there are now multiple "universal" markup programs which really defies the whole purpose of having a universal language I think.