I sort of lament the use of lines under headings, as they look visually better for human readers of Markdown, but they're worse in other ways: ambiguous heading levels (quick, which is h2, `===` or `---`?), and less token-efficient.

For me, the use of indentation over ``` for code blocks is a complete non-starter, though. It's hostile to copy-and-paste.

> I sort of lament the use of lines under headings, as they look visually better for human readers of Markdown

Maybe I am spoiled but I rarely read markdown directly. As a user of vscode I do ctrl + shift + v on a markdown file and read the rendered version.