But isn’t the whole point of the standard that previously, there was no one way Markdown parsers worked? That everyone interpreted the original description differently? What exactly is a parser in quirks mode then supposed to do differently? Isn’t standard mode just another acceptable way a parser would behave under the old interpretion?
To me, a different mode and validation only makes sense once the standard introduces breaking changes from the status quo. But as far as I understood the whole Standard Markdown project, its primary purpose is to find a common ground of all the different parsers.