Also, your first sentence is asking the wrong question ;P
(immediate "tags" that my brain went to upon seeing this: printer mode, accessibility modes for visually impaired, text-to-kindle, text-to-audio)
That's not what this is. Try copying the markdown so you can edit it as you suggest and you will find it doesn't work. The output looks like markdown, but it isn't markdown, and that's the source of the negative reactions. Having a one-to-one correspondence between representation and rendering is the main thing that makes markdown useful. Making something look like markdown but not actually be markdown strips it of nearly all of its utility.
And looking simple with minimal formatting is the second thing. This idea taps on that second thing.
1. Take some HTML
2. Use CSS to make it look like Markdown
Why not skip step 2?
Edit to add: Not suggesting that you think it needs to serve a purpose, this just felt like the most appropriate thread to add my comment to.
Personally, I'd consider using this for projects where I want the plaintext aesthetic, but also with the semantic elements that HTML provides for accessibility/interactivity purposes.
Here are some other things I’ve built in the past because the code and/or the idea was interesting to me: http://mrcoles.com/failed-projects/ | http://mrcoles.com/dragondrop/ | http://mrcoles.com/tetris/ | http://mrcoles.com/low-res-paint/ | http://mrcoles.com/piano/ — don’t bother asking for a usecase ;)
For example, writing " * this is highlighted * " would be shown as " * this is highlighted * ".
Edit: I've just found http://hackcss.com/ in the comments below, which seems to get it and does what I've explained above, highlighting headers and code with a bold, darker font.
I'd still add a mode like Microsoft Word's "show formatting marks", so that the user can decide whether to see the full markdown text or only the word or line being edited.
That seems broken.
That said... why not Pandoc? Just too heavyweight for a browser?
It should turn a line break into br, then markdown would be alright.
The problem is many input fields in websites trending to use it now. I think BBCode was more suitable for that. Markdown is easier than BBCode on one hand, but the newline thing ruins it for simple replies to posts.