> I'm 99% sure that Gmail will still send a plaintext e-mail inferred from your HTML content whenever you send, so it's not quite accurate to say it doesn't send in plaintext.

I think that might be possible in the "old" gmail, I was referring to their newer "Inbox by gmail" client. I believe it never sends a text/plain email regardless of content (And it being their "newer" revamped client kind of shows what googles take on this is...)