One can use \newcommand with arguments for editing HTML today. This part of GELLMU is now stable -- except that there may be further enhancements to \newcommand. There is no limit to the number of \newcommand arguments.
While the use of the GELLMU syntatic translator (which resides in the Emacs Lisp program corresponding to the file gellmu.el) for editing HTML comes under basic GELLMU, and in some sense, is a side show compared to the program for regular GELLMU, it is nonetheless a way to begin to become familiar with the way that LaTeX-like markup can be used to make HTML and other document types under either SGML or XML in a completely reliable way. Of course, one does need to know the vocabulary of that document type.
Substantial customization of the syntatic translator is required to bring regular GELLMU to bear on a document type other than the GELLMU didactic document type. See the variables in the Elisp variable "gellmu-public-vars", which are the variables open for user configuration.