More recently, purple text has been proposed; smilies and tildes are used on occasion. (As if!~)*
But the Dutch had their own idea; in 2007, the ironieteken was put forward with great fanfare during the national book festival.
It looked like this:
A flash of lightning! A bolt from the blue! The sting of snark, the bite of sarcasm...!
And then someone with a piping-fresh font in their computer typed two of them side-by side. The thud of of jackboots couldn't have been any louder. My, how terribly familiar....
The ironieteken died a quiet, hasty death, unmourned.
And you're still stuck figuring out sarcasm and its kin on your own. It's a kind of ongoing Voight-Kampf test.
How'd ya do yesterday?
* A bit of a problem for me, as I habitually use "~" to indicate "approximately," as in "~100 miles from here to Dayton." 13 years of formal education, and I came away with that.