> In fact, a code fence need not consist of exactly three backticks or tildes. Any number of backticks or tildes is allowed, as long as that number is at least three

Unfortunately, some markdown implementations don't handle this well. We were looking at using code-fence like syntax in Rust and we were worried about people knowing how to embed it in markdown code fences but bad implementations was the ultimate deal breaker. We switched to `---` instead, making basic cases look like yaml stream separators which are used for frontmatter.

All this complication seems to stem from the simple fact, that the fences don't have a recognizably distinct start and end marker. It's all "`" or "~", instead of one symbol at the start and another, different symbol at the end. And then going into the different numbers of backticks or tildes. Why add such ambiguity, that will only make it harder to parse things correctly? This immediately raises the question: "What if I start a block with 4 backticks and end it with 5?"

All these complications would have been avoidable with a more thought through design/better choices of symbols. For example one could have used brackets:

    [[[lang
    code here
    ]]]

And if one wanted to nest it, it should automatically work:

    [[[html
    html code
    [[[css
    css code
    ]]]
    [[[js
    js code
    ]]]
    html code
    ]]]

In case one wants to output literally "[[[" one could escape it using backslash, as usual in many languages.

In a parser that would be much simpler to parse. It is kind of like parsing S-expressions. There is no need for 4 backticks, 5, or any higher number. I don't want to sit there counting backticks in the document, to know what part of a nested code block some code belongs to. It's a silly design.