Hello everyone!
This thread has raised my attention and I would like to share my 
opinions, maybe as a "school child" who used mathematical software for 
WYSIWYG editing (not only reading!), as the primary way of editing any 
math, as a primary/fundamental tool for computer-aided learning. I was 
(un)lucky enough to be forced by my situation to learn using *only* 
computers in the late 1990s and early 2000s. That experience has taught 
me the importance of WYSIWYG editing for HTML and maths.
I feel it's not easy to me to reply to this thread - seeing other people 
who are technical experts that I admire have already replied, providing 
proper arguments for their reasoning. Please excuse my, perhaps, less 
formal, less backed-by-arguments reply.
This thread shows that there's some misunderstanding on the performance, 
styling and editing requirements for math. I can say that I spent months 
trying software to find the best one fitting my requirements. It wasn't 
easy.
I haven't seen good (La)TeX WYSIWYG editors, but lately I haven't tried 
any such software - now I write LaTeX manually. Still, in the early 
2000s I did see and use one WYSIWYG editor that was really good: 
Wolfram's Mathematica. It had fast rendering, good set of keyboard 
editing shortcuts allowing fast input in WYSIWYG mode. Really good math 
WYSIWYG editing is very much possible.
Performance matters not only for the initial document rendering. When 
you do WYSIWYG editing performance characteristics matter in a lot more 
subtle ways. When you are editing big equations, or some really big 
document updates need to happen as close as possible to instant. I have 
tested software like MathCAD and Maple that did not seem slow at all 
when loading documents. Editing math, however, proved to be quite slow. 
Very good editing is *not* about "click and point" - this was one of the 
biggest failures of MathCAD's UI: it encouraged the click-and-point 
editing which meant you had to switch between the keyboard and the mouse 
all the time. Word 97 (before Word 2007) forced you to manually switch 
between the equation editor and the normal editor, which was a huge 
problem, and so on.
Styling is really important when you collaborate with others and you 
need to highlight relevant parts of the math output. I am surprised this 
is even put up as discussion.
Similarly I am surprised that the need for WISYWG editing for math is 
being discussed. I am being subjective here: I believe that mathematics 
should be first-class citizen on the web. Mathematics is a fundamental 
domain of study in all schools, in all forms of education throughout the 
world. Mathematics is the basis for many other fields, see physics, 
computer science and others.
Back in those days when I was writing math homeworks with Mathematica I 
was very glad and I appreciated a lot that people write software that 
can benefit my niche needs, it was invaluable for me. It made possible 
things that were not possible. Microsoft's Word was not even close to 
being as usable as Wolfram's software. Word 2007 has, indeed, improved 
math editing a *lot*, today it's certainly usable.
Microsoft's work on improving math editing in Word shows there's a real 
demand for math in documents. I don't see why we would believe otherwise 
about the web. We should not need to include half-baked* JS libs to 
render math in a document.
* I'm not claiming that MathJax is half-baked - I am simply pointing out 
that once people have the choice of which JS lib to use for math 
rendering they may (and will) fail to pick the best one.
I do not care about the technology here - MathML or TeX. What I care 
about is for the web browsers to meet the technical demands for 
producing really good math rendering and editors. I want this not for 
the academics, not for professors who can write TeX documents. I want 
this for school children who cannot write math on paper, who are blind, 
or who have other physical disabilities. Manually writing LaTeX does not 
"cut it" at early stages, when children learn maths. Such tools are 
invaluable for them.
At the moment, removing MathML support from Gecko would make it harder 
for web app developers to create (really) good software for math 
editing. It may certainly have its problems, but its benefits are 
greater. Before MathML is removed people should look into defining the 
requirements, the APIs needed to be implemented in the browser such that 
JS-based math rendering can be equally fast and versatile (eg. styling). 
Font metrics stuff is, I believe, only a part of the problem that makes 
JS-based math rendering slower than native. After requirements are 
defined, those things should be implemented. After that, yes, remove MathML.
Back in the days when I was testing math software, I was also testing 
MathML rendering in Gecko - it was slower than in specialized software. 
I don't know how it is today, but keep in mind that native software like 
Maple and MathCAD was not usable due to performance issues, during fast 
editing of small to medium sized documents. It may take some time before 
web apps can become as fast as Mathematica at rendering math, and as 
good at editing -- even with MathML rendered natively.
Editors are really hard and it is unfortunate to note here that browsers 
do not even do good enough at HTML editing. If we can do something to 
improve the situation we should do that - not the opposite. The removal 
of MathML would most-likely make things worse/harder for web-based math 
editors.
Probably there is not much "value" from maintaining MathML - browser 
competition happens in other areas, other APIs and technologies. 
However, please let the volunteers do their work, maintain their work 
and so on. From reading this thread I understand MathML support in Gecko 
was implemented mostly by volunteers. It would be a big disappointment 
to volunteer efforts to see that work goes away, especially without 
anything better replacing it.
I doubt that if we keep MathML some day some people would like their own 
niche markup language - eg. for domains like chemistry, biology, music, 
etc. Did you see anyone doing that?
I find it surprising that HTML5 caters to advertisers/trackers by 
introducing the ping attribute for anchors, yet here we question the 
use/need for a standard way to write mathematics on the web - the 
initial email in this thread questions the need for anything to replace 
MathML, as writing maths is over-specialized.
Thank you for reading. Feel free to take these thoughts with a grain of 
salt: I am biased, I was a user of native math software and I would like 
the web platform to provide equally good software.
Best regards,
Mihai