It would be useful to be able to post e.g. our transcription attempts for more experienced site members or admins to review without automatically spoiling them for other people working their way through the lessons, and I didn’t see any way to easily do that with the formatting options displayed in the default editor.
Enter StackOverflow to the rescue - see below for an example.
Using the "details" and "summary" HTML tags, text can be hidden until the user clicks on the element, just like a spoiler tag! (click to expand)
If you can see this only after clicking, it worked!
Here's a code block - might be useful for formatting tabs, rhythms, etc.
To create a spoiler tag using HTML:
<summary>Always visible text goes here</summary>
Hidden text goes here.
EDIT: As pointed out by the knowledgeable stitch, click the gear/settings icon in the editor and click Hide Details to automatically insert the correct HTML.