Well the biggest concern that other people had with --- for strikethrough (proposed before) that made me add >><< is that they said that there is a danger that the syntax might clash with the horizontal rule.
Also how do you tell if the person wants a trailing space, e.g. === this has a trailing highlight===. But then again… unlike reviewing markup syntax… it is unlikely that missing a space to be a big deal. Well I guess it can work, if we adopt the same rule as the bold syntax, and require no space. This is so we can deal with people typing stuff like --- as an em dash.
Amended Proposal
--striked through text--
==Highlighted Text==
__Underlined Text__
>>! Spoiler Text Here !<<
for empty underlined fields (e.g. for signatures)
>>________________<<
edit: I think to be on the safe side, we should require three characters
edit2: scratched that… its annoying typing 3 chars actually. Hope it’s still safe.