It's common practice when dealing with sites and clients that don't have fancy quoting features, going all the way back to USENET forums and probably before. It avoids just this ambiguity when you might be mixing quote and commentary.

Hmm, honestly I’ve mostly seen > used for quotations in plain-text-y environments. Not sure about USENET, but ever since email it seems to be the de-facto standard everywhere. (On HN, I mostly see >, italics, or monospace as the quotation indicators.)