My sense is that Markdown is
probably pretty safe for most uses, particularly if you control the processing. If not, then yes, it can bite. For me that means pandoc to generate endpoints such as HTML, PDF, etc. I'm fairly confident that most of that toolchain should continue to work (provided computers and electricity exist) for another 2-4 decades.
For certain more complex formatting, Markdown has limitations and features are more likely to change. But I've used Markdown to format novel-length works (from ASCII sources, for my own use) with very modest formatting needs (chapters, some italic or bold text, possibly blockquotes or lists), and it excels at that.
For HTML, it's a combination of factors:
- Previous features which have been dropped, most to thunderous applause. (<blink>, <marquee>, etc.)
- Previous conventions which have largely been supersceded: table layouts most especially. CSS really has been ... in some respects ... a blessing.
- Nagging omissions. The fact that there's no HTML-native footnoting / endnoting convention ... bothers me. You can tool that into a page. But you can't simply do something like:
<p>Lorem ipsum dolor sit amet.
<note>Consectetur adipiscing elit</note>
Nulla malesuada, mauris ac tincidunt faucibus</p>
... and have the contents of <note> then appear by some mechanism in the rendered text. A numbered note, a typographical mark ( * † ‡ ...), a sidenote, a callout, a hovercard, say.
In Markdown you accomplish this by:
Lorem ipsum dolor sit amet.[^consectetur] Nulla malesuada, mauris ac tincidunt faucibus
[^consectetur]: Consectetur adipiscing elit.
Which then generates the HTML to create a superscript reference, and a numbered note (when generating HTML). Or footnotes according to other conventions (e.g., LaTeX / PDF) for other document formats.
- Similarly, no native equation support.
Maybe I'm just overly fond of footnotes and equations....
But HTML and WWW originated, literally, from the world's leading particle physics laboratory. You'd think it might include such capabilities.
- Scripting and preprocessors. I remember server-side includes, there's PHP, and JS. Some browsers supported other languages -- I believe Tcl and Lua are among those that have been used. Interactivity and dependency on other moving parts reduces reliability.
The expression "complexity is the enemy of reliabilty" dates to an Economist article in 1958. It remains very, very true.
HTML is for me more fiddly than Markdown (though I've coded massive amounts of both by hand), so on balance, I prefer writing Markdown (it's become very nearly completely natural to me). OTOH, LaTeX isn't much more complex than HTML, and in many cases (simple paragraphs) far simpler, so if I had to make a switch, that's the direction I'd more likely go.