May I recommend ConTeXt?
For my purposes, KeenType provides a real-time "rough draft" of equations rendered inside of KeenWrite[0], my FOSS Markdown editor. Rather than reinvent the typesetting wheel, I edit in Markdown, then KeenWrite converts the inline TeX-based equations for previewing using KeenType. When I'm ready to create a "finished" document, KeenWrite exports the Markdown to XHTML then feeds the XHTML, along with a theme, to ConTeXt[1]. The various themes[2] are where font faces sizes, alignment, kerning, line spacing, headers, footers, margins, etc. are tweaked.
In this way, content remains completely separated from presentation logic.
As a side note, KeenWrite 3 (and newer) automatically requests to install both ConTeXt and KeenWrite Themes when a user first exports a Markdown document as a PDF file.
[0]: https://github.com/DaveJarvis/keenwrite
If it doesn't support kerning, it's not really a typesetting system. I would call this a "mathematical formula editor" but not a "typesetting system."
To be more conformant to my expectations for a typesetting system, I'd suggest to add more information to the docs so you can understand how to specify things like font face, font size, horizontal and vertical element positioning and line spacing, etc.