Editing equations after HTML output

The characterization of the output suggests that what's being used is the legacy FM Equations editor, which has the workflow limitations observed.

Until some time in the recent past. the newer way to handle equations was via the hooks that FM provided to MathFlow (a 3rd party MathML editor, later renamed to MathType) apparently no longer supported for FM. If you already have MathFlow working, you can use it to get equations rendered to HTML as XML. If you don't have it working, it cannot now be turned on (Tracker FRMAKER-11189).

Here's some back forum traffic on the matter:
Design Science is no longer selling MathFlow, is there another way to use MathML in FrameMaker?

Word/MathType to FrameMaker/MathML Equation

If I had a requirement include complex Eq in a new document, I'd probably take one of two approaches:

1. Use an external MathML editor to create an XML object. Import that.
2. Use an external equation editor to create the equation. Save as PDF or SVG. Import that.
   Either would at least preserve high vector quality.

You can try using MathType, which is a 3rd party application. I believe the equations are maintained in XML in the output.(probably SVG, which is an XML standard, but I don't have MathType installed at the moment)

Also, I believe anything that is SVG should remain SVG in the output, and thus be editable.

So, any graphic from an application capable of saving as SVG should remain SVG and be editable in the output.

Why do you want to edit the HTML output instead of editing the equation and then reoutputting the HTML?

If you decide to use something "outside" of FrameMaker's editor, here's a video showing improvements in SVG handling from Fm 2019

https://www.youtube.com/watch?v=Oowg5mX4LeQ