Oh hey, we found the "show all email in plain text unless I say differently, dammit!" setting. Turns out there was a section of options I didn't capture because there was no earthly reason to think there was anything relevant there. It's in Options -> Trust Center -> Email Security -> Read as plain text. Because sure, I should think of this as a security issue and not an accessibility issue! (Doesn't Outlook automatically not run scripts embedded in HTML, anyway? Even the web client, which uses HTML, makes me click to see some content.)