There's a very small set of text decorations which would suit virtually all cases.
And there was a long history (about 100 years) of monospaced, mono-sized, typographic conventions based on typewriters and cheap reproduction (carbon paper, spirit duplicators, xerography), before the desktop-publishing era brought us out of that in the 1980s. The problem with formatting is that, like Johnny Rocco in Key Largo, someone always wants more, and not for your benefit but theirs.[1]
In the Web world, counter to an argument elsewhere in this thread (https://news.ycombinator.com/item?id=27113404), my experience is that Web design isn't the solution, Web design is the problem. Straight ASCII text (via w3m or other text-mode browsers), or Reader Mode (where supported) is vastly preferable to many, many nines worth of sites' prescribed designs.
One thought I've had is that user agents could provide, at user discretion, additional typographic control to sites. But for the basics you're limited to 7-bit (not 8) ASCII. Everything else is earned, and you'd best degrade very gracefully.
I've seen websites written where every individual paragraph has an explicitly-specified location. I've seen blogs written entirely in header-level tags. I've seen writers who add nonbreaking whitespace to the start of every paragraph. I've seen blink, marquee, and carousel abuse. I've seen foreground and background colours you wouldn't believe. Attack ships on fire off the shoulder of Orion....
I would be fine if my client would neatly format a limited markdown format and other clients supported entering it in a WYSIWYG fashion, as long as all the displayed content is part of the transmitted message (no further connection to the net) and the client is guaranteed not to execute any sender-supplied code or transmit any data when links or images are displayed.
Making it unobtrusive enough to still look good when uninterpreted is a good goal. But that's very different from having nothing at all.