Earlier quoted context omitted.
That’s because the HTML renderer in Outlook is actually Microsoft Word, which is painfully ridiculous and something only a monopoly could get away with.
Why!??? Doesn’t Microsoft have 10 other HTML renderers they could use?
I'm as dumbfounded as you, honestly. Even the IE11 engine would be a step-up to be honest.
In other systems, we are still required to use HTML4 Transitional, and Outlook is only part of the problem. Even webmail, which you expect to handle modern HTML, just fails on unexpected ways (no CSS on head, even if it's embedded on the mail, is one of those egregious problems). It's miles better to be the one working on the website than the marketing email.