I was thinking about starting this anyway, but then I saw this was listed as an appropriate topic in the pinned feedback thread.
It’s a big change, but Standard Markdown would be immeasurably helped by abandoning indented code blocks. They’re a terrible, inconsistent part of the original Markdown, which causes problems throughout the rest of the language, and fenced code blocks are better in every way.
For one, indented code blocks mean that you’re never allowed to indent markdown for any reason, which is pretty limiting. I’m a CSS spec author, and write my specs with a processing tool called Bikeshed, which partially supports Markdown. It’s common convention in Bikeshedded specs to only write headings against the margin, and indent everything else, as it makes it easier to quickly skim through a big source file and see where the sections are. Here’s an example source file.
For two, indented blocks make handling Markdown nested in HTML much more difficult. Bikeshed supports intermixed HTML by detecting lines that start with a block tag, and outputting just that line literally; the very next line can be (indented) markdown text again. I suspect the reason SMD has the slightly odd rules about HTML blocks is to avoid accidentally triggering code blocks from the indentation. Bikeshedded specs regularly put Markdown text on the very next line after an HTML tag, and it looks completely natural, so this is a place where I’m currently going to have to willfully violate the spec.
For three, the handling of indented code blocks within lists is crazy town - having to indent 8 spaces makes it look terrible.
For four, blank lines in code blocks are hard to see/express. Plus, some editors have options to auto-remove trailling whitespace from a line (I use this, to reduce spurious diff churn). (On that note, I’m glad to see a way to force hard breaks without trailing whitespace make its way into SMD.)
Overall, if indented code blocks go away, Markdown can become completely indentation-agnostic, where the only real requirement is that following lines in a block have to be indented at least as much as the first line. That’s basically what I’ve implemented in Bikeshed so far, and I plan to continue supporting precisely that, even if I match SMD everywhere else.