[MatmaRex]

> First, it relies on the function Wikitext->HTML being injective. But isn't it trivial to create two different Wikitexts that compile to the same HTML? Whitespace is just the start of this story.

Yes, and Parsoid works around this by preserving some metadata about wikitext in HTML (such as information about whitespace around syntax elements) and, since this preservation isn't perfect, only reserializing HTML→wikitext where the content was changed during editing.

> Second, apparently the template language is Turing-complete. Let's say I write a prime sieve in order to generate a page that lists the first 100 prime numbers. What would it then mean to edit "31" to change it to "30"?

Assuming we're talking about VisualEditor, you currently just can't do that (you can only delete the entire template inclusion and replace it with normal text, or edit template parameters).

(As a nitpick, wikitext is not Turing-complete (there is no loop or recursion construct, recursion is explicitly checked for and causes an error), you can only write complicated algorithms by manually unrolling enough loops. However, for some time now you can also write templates in Lua, which is a proper Turing-complete programming language, see <https://www.mediawiki.org/wiki/Extension:Scribunto>.)