This is more akin to the old Adobe MultiMaster fonts, where the user installed one font that could then be dynamically tuned in various ways - weight, descenders, etc.
The book Computer Modern has a fun section at the end where it goes through several poems showing different parameters for the font for each section.
But it seems that it ended up being used for a much more narrow purpose in the actual TeX stack that people use.
I hate the conservative, everything is old and has been done view; however, it is easy to look at some of the original large projects that people chose to ignore and be amazed at what they were already doing. It isn't in XML, but METAPOST/METAFONT are two really really cool projects.
yeah, well, same true for Multiple Master fonts. It ended up that Adobe gave the font metrics to the TeX[0] community. The complexity of the fonts was obviously too much for the "graphic designer".
My first year of college was the first year my school shifted most departments' document recommendations to MS Word from TeX, so I never really used it for authoring. Spending a day playing trying to make something useful with it has been on my someday-list for... a depressingly long time.