This is an enhancement or feature request.
"What is the limitation or missing functionality?"
Currently there is a parser function documented here - https://www.mediawiki.org/wiki/Help:Magic_words#Parser_functions
called #tag which generates a balanced pair of tags for HTML or pseudo tags.
However it's not currently possible to split a #tag function, for situation when only one half of a pair of tags is needed. Wikisource uses a number of templates which are currently directly coded as /s /e versions containing the start or end tags respectively.
"What is the desired functionality?"
Implement an #open-tag and #close-tag function to generate the start of or end of a tag pair respectively...
- Do any pseudo tags implement attribute in their closing tag?
- Some pseudo tags are in effect self closing like <section> tags used for Labelled Section Transclusion, without intervening content.