There was an attempt once to create a standard to describe/evaluate CMS. Here’s an article from 13 years ago where Erik Hartman proposes Content Management System Markup Language (CMSML).
The advantage of CMSML is the use of a certain “controlled” vocabulary that is supported by an ontology. Furthermore CMSML is a faceted classification system, which means that people can select products based on their own characteristics or properties.
The idea was to allow comparison of CMS products, but I think a more valuable usage (today, at least) might have been for API detection when writing headless middleware – clearly something not on the radar in 2005.