All of them use some sort of Webbrowser underneath. Thunderbird is using Firefox/Gecko, Geary is using GTKWebkit, macOS Mail is probably using Webkit. Note sure what Windows Mail is using. (Similar to outlook in my experice)
Outlook... is using Microsoft Word. (I mean technical it's just Offices' document rendering engine, but for meail it's equivalent as opening a HTML file in MS Word) (Also sends RTF mail for addresses on the same Exchange server)
Although I'd note that they strip down the HTML avialible. Emails can't execute Javascript. CSS is stripped down and must be in file. (Or completely inlined)
You limited to some very old Web technology in email. (Unless Google somehow managed to convince anyone else to support AMP 4 Email)
Outlook... is using Microsoft Word. (I mean technical it's just Offices' document rendering engine, but for meail it's equivalent as opening a HTML file in MS Word) (Also sends RTF mail for addresses on the same Exchange server)
Although I'd note that they strip down the HTML avialible. Emails can't execute Javascript. CSS is stripped down and must be in file. (Or completely inlined)
You limited to some very old Web technology in email. (Unless Google somehow managed to convince anyone else to support AMP 4 Email)