As the main offender, let me try to give an explanation.
For the sake of full disclosure, this is the - admittedly canned - comment, that was supposed to be in the edit summary:
I have updated your post with chemistry markup. If you want to know more, please have a look here and here. Please do not use markup in the title field, see here for details.
Now I have used this comment, because this is essentially what I did. However, that was not the original intend, but more to this later. Let's work our way up the ladder.
Is HTML banished?
In the most general sense: No. But I would hardly consider it a recommended practise to use it instead of MathJax, which I would. That being said, I thank you for bringing this to meta, because I am of the opinion, that any kind of mathematical and/or chemical markup should be handled with MathJax.
The main intent was apparently throwing away HTML formatting and rewriting it in mhchem.
That was not the main intend, that was just a by product. I thought this posting was well enough to deserve my up vote. However, I noticed a small typo. Instead of just correcting that I introduced MathJax for the sake of consistency.
The second point that I wanted to correct were the emoticons. I consider them noise, they distract from the point of the post and to be perfectly frank have no place in the scientific world at all. If it was not for the smileys, I would not have bothered to alter any of your post.
Unlike customs at most SE sites, editor provided me with a message.
As most of the moderators across the network I always encourage other users to enter a edit summary, even if it is just a canned one. If you have made negative experiences so far, I am very sorry to hear that.
But the message seemingly consists of some boilerplate phrases unrelated to specific case. Link to posts at Meta discuss such scientific typesetting basics that I obviously know (Roman type for elements and units, etc.).
It was in no way obvious, that you already know this. These posts essentially state what most of us consider good practise. There has not always been consensus about it, but after some time and discussion, we agreed on that. I think it is nice to know; it also serves as somewhat a reference. If you already do know these point, you can just ignore this advice.
Mhchem is an extension of MathJax, which works only in modern browsers (and heavily uses browser’s resources, by the way).
I think if you are still running a browser that is not capable of running MathJax, you have more serious problems than caring about that you cannot read the formulae.
It is true that MathJax heavily uses the browsers resources, it will be loaded in any case, even when not in use and will use some of these resources. Actually using the renderer might increase that load, but something as simple as $\ce{O2}$ should not be a big problem and not cause much more usage than O2.
And AFAIK its code isn’t well-searchable, also.
I think you are mistaken here. From the documentation of MathJax:
With MathJax, mathematics is text-based rather than image-based, and so it is available for search engines, meaning that your equations can be searchable, just like the text of your pages.
How it performs compared to HTML style equations I cannot comment on, but I would assume, that it still performs better. I would assume that when searching for H3PO4 I would find \ce{H3PO4} but not H<sub>3</sub>PO<sub>4</sub> or even the unicode string H₃PO₄.
Was that important to litter with $…$ a post that happily lived without MathJax at all?
I would not consider it littering, but of course it was not necessary, as I already stated.
Let aside such minor damage as incorrectly capitalized word “hydrogen” after the colon character, [...]
I am not a native speaker, so I apologize introducing mistakes into your posting. I was not aware of this rule. Thank you for pointing it out to me, I will pay more attention next time.
[...] typewriter apostrophes, [...]
That was something my spell check found, I thought this to be correct, again I apologize if this is actually wrong.
[...] and disappeared emoticon.
Well, we covered that already, but again, that was the whole point of the exercise.
Of course, if there is a generally discussed agreement upon HTML, some guideline, then just point me to it. But if there is no such agreement… a usual convention is precedence of original poster’s style over subsequent editor’s style. Again, if the former isn’t discouraged site-wide. Or may anybody at chem.SE edit others’ posts for no pretext but to change style?
I believe there is no such general agreement other than the recommendations, which are already linked. There had been a discussion about it some years ago, but it was more about the style within MathJax and the enabling of mhchem. When we were still in beta, we had a sans serif font which did not harmonize with MathJax. Since graduation the problems stated here are basically gone.
In MathJax Buttons on stackapps you can also find the following statement, which I think is true for most of our users:
Well, many SE sites have mathjax built in soit's easier to just use it. In fact, we convert subsups to mathjax--it's easier to read. Since everyone is using MJ anyway, it's better to make life easy for them. And trust me, these have been extremely useful to me on chem.SE :) – Manishearth
Other than that, you are of course correct, the style of the OP should be preserved. That being said, you always have the possibility to roll back the edits to your version, if you cannot in good faith agree with the edit at all.
And since this is meta: Thank you for your attention :D
