(the short answer)
If you want to leave a comment on my blog fancier than plain text, probably whatever you try will work. Markdown will probably work, and reasonable HTML will probably work, and if you invent your own markup notation it might just work.
(the long answer)
What magical software is so universal and miraculous? The answer is actually that my blog runs on the least magical, universal, and miraculous software possible. There is no real comment system. When you click "submit a comment", I get a message with what you wrote, and I add it to my website manually when I get around to it. This means that you can even describe the formatting you want in English, and I will simply do it myself.
Of course, I will only do this for sensible things. Bold, italics, underscore, strikethrough, color, alignment, and so on are all clearly fine. Font size and similar also are to an extent, but not if you ask for 6000pt font that covers the whole screen, for example. At the other extreme, an embedded script containing a Bitcoin miner will obviously not make it through. What about the middle? If you ask for something very weird (e.g. for a game of Minesweeper to be embedded in your comment), whether that happens will be highly variable, depending on how harmful it is to visitors to my site, how difficult it is to implement, how lazy I'm feeling at the time, how funny I think it would be, and so forth. Try this at your own risk, since if I decide it's too much work I might just give up and ignore the comment entirely.
The manual process means your comments might also be subject to minor editorializing. For example, if you leave a perfectly reasonable comment follow by the entire text contents of Atlas Shrugged by Ayn Rand, I will only post the reasonable part.