FWIW, WordPerfect's "reveal codes" feature remains my ideal solution.
Long before the advent of desktop publishing and GUIs and markup languages. Split screen horizontally, vs vertically side-by-side, with WordPerfect's own nomenclature for formatting attributes. Edit in either screen. Live cursor shown in both screens. Live updates back and forth.
Heaven.
I haven't used WordPerfect for decades, so my bias might just be nostalgia.
Slack's "markdown" is horrifically broken and one of the worst markdown experiences in existence. I believe they no longer even refer to it as markdown since they have strayed so far from the spec.
This title belongs to ClickUp.
Bold, italics, links, etc. -- sure, false positives there are rare enough and the means of correcting mishaps are intuitive enough. The OL issue though... so many go uncorrected.