Is there a good description somewhere of HTML equation writing and how to use it? I'm no HTML whiz, and the problem I usually run into is that the standard only defines things in a way that is ultimately correct and definitive, but really hard to parse. Other places give you small snippets at a time and are more focused on how to get something done instead of explaining how things work. I'd like to read the 10-page or so description of how the math engine works and what the tags do, without it being fragmented into 50 three-second sound bites you have to individually click thru.

The nearest I could find to a basic overview is Getting started with MathML, which gives an idea of how things nest inside each other to give different arrangements of symbols on the page.

For occasions when you just want to get the relevant markup to paste in, you can write it in MathJax on a Codidact site that supports it and then in the preview you can right click on the resulting mathematical notation and select "Show Math As > MathML Code". You don't even need to save the post - once you have copied the MathML from the preview you can discard the draft.

I never did learn MathJax, so figure I'd just learn to use the HTML math features directly, assuming they aren't too difficult. I don't use Markup either. The HTML it replaces is hardly more complicated than Markup, and I can then use it in any HTML document. Since MS Word has gotten ever harder to use with each release, I've lately been writing documents in HTML. Much of what I do doesn't have Markup or MathJax available, so learning the direct HTML is useful. It's really not that hard. It reminds me of using a "runoff" style program from the 1980s, except that I can see the result immediately in another window where the browser is displaying the document I'm editing in a text editor.

For someone who isn't already accustomed to MathJax, the easiest way might be to find something similar to what you want in an existing post and copy it as MathML and edit that to suit your needs.

Since you're already comfortable with HTML you'll probably find MathML sufficiently similar, especially when you can see the output immediately and get a feel for what each change does to the layout. The only potential problem with that is that you may end up doing things in a way not intended by the standards (similarly with HTML). Whether this matters to you depends on your intention. If you're writing a paper you might care more about precise layout than in an aside where the mathematical notation is not the main focus.

For example, the following all display as "1+1=2" but the spacing is different:

<p>1+1=2</p>

<math>1+1=2</math>

<math>
  <mn>1</mn>
  <mo>+</mo>
  <mn>1</mn>
  <mo>=</mo>
  <mn>2</mn>
</math>

The third one is the standards compliant way.

For more complex expressions accessibility becomes relevant because assistive technology such as a screen reader might not know how to describe non-standard usages.

I just took a quick look at the MathML tutorial you linked to. It doesn't look bad at all. Granted, I haven't tried to write a fancy equation with it yet, but the basic concept seem straight forward and reasonably well thought out.

One thing that bothered me in that writeup is how they say you have to be careful with the font and code pages. Surprisingly, there was no mention of using HTML entities, like &, Ω, Θ, etc.

Indeed. Similarly to HTML, you can get started before you learn everything. You can use HTML to write a blog post without having to learn how to make an HTML table with merged cells. In the same way you can write clear, readable fractions in MathML without having to learn all the obscure notations of niche fields.

I think the problem they are describing with fonts is that even if you know the correct HTML entity for something (like & for &) the fonts present on your system may not cover all of the obscure symbols you might want to use, so the HTML entity will just be displayed as a meaningless box like □.

This won't be a problem for common symbols that your system already has fonts for, but if you're not sure if your reader's device will already have a font that can display for example ℕ as ℕ then you might need to include a font file in your page.

Apart from such obscure cases, if you're used to using HTML entities then MathML may be quite natural for you. In MathJax, you would use \Omega or \Theta for Ω or Θ (or \omega or \theta for lower case ω or θ). In MathML you would use the HTML entities Ω, Θ, ω, or θ.

I found a list of mathematical HTML entities.