There are a large number of errors where the New Wikitext Editor preview does not match the genuine article view after save. Discussions on projects pages indicate that some of those rendering errors are explicitly CANTFIX/WONTFIX, that the WMF plans is to play whack-a-mole on endless individual rendering errors, and that the WMF is explicitly aiming for less than 100% accuracy due to CANTFIX/WONTFIX issues. These issues appear to be due to using the VE engine (Parsiod etc.) to generate previews, rather than using the genuine article view engine (PHP parser etc.).
The task is for previews to use the genuine article view engine (using PHP Parser etc.), like the current wikitext editor, fixing all of these errors at once.
**Strong community consensus that this issue is serious enough to be a deployment blocker**: [[ https://en.wikipedia.org/w/index.php?title=Wikipedia%3AVillage_pump_%28proposals%29&type=revision&diff=771419642&oldid=771287413 | EnWiki Village Pump RFC is here.]] The key text was:
//Actionable Blocker: Article previews. Previews are a critical part of the learning process for new editors, as well as a critical tool for experienced editors. Inaccurate previews will confuse, disrupt, and frustrate. Possible resolutions://
* //We could happily retain our current wikitext editor; or//
* //previews must use the article-view rendering engine (this is called the PHP parser), as well as any other relevant parts of the article-view software stack. This fixes the assorted rendering errors all at once, and it ensures that previews remain synchronized with article views when any changes are made to the software.//