Steps to reproduce: enter – in the wikitext of the page
Expected result: an en dash (–) is displayed.
Actual result: – is displayed (on the HTML level, & gets encoded into &.
The relevant spec is consume a character reference in HTML5, which has a compatibility table for frequently used character codes which are not Unicode code points. (En dash is U+2013 so the straightforward representation is – but the HTML5 standard also acknowledges Windows-1250 codes for a number of characters which when interpreted as Unicode would result in unprintable characters - U-0096 is "start of guarded area".) MediaWiki should respect that compatibility table when encoding references.