The HTML5 standard says that the XML-ish self-closed tag syntax <TAGNAME/> (note the trailing slash) is ignored: tags are "self-closed" iff the tag name matches a list of "void tags". The only valid void HTML tags are area, base, br, col, embed, hr, img, input, keygen, link, meta, param, source, track, wbr
As such <b/> and <span/> are treated exactly the same as <b>, <span> in a HTML5 parser. But, the situation is a bit complicated in mediawiki.
- Without tidy turned on, the Sanitizer mostly enforced this constraint but rewrote <b/> as <b/>. That isn't strictly according to the HTML5 spec (which would rewrite it as <b>) but does get the point across that this is invalid HTML syntax.
- When tidy is enabled, tidy replaces <b/> with nothing, that is, it removes the invalid tag from the output. This has led to its (ab)use as a way to protect leading/trailing whitespace and punctuation in templates. However, there are alternative ways to do this, including <nowiki/> and  , which don't violate the HTML5 parsing rules.
- However when we replace Tidy with a HTML5 parser (See T89331), Mediawiki will start enforcing the HTML5 standard and parse <b/>, <span/> as start tags which can break rendering on pages that might (deliberately or accidentally) rely on Tidy removing these tags.
In order to facilitate a smooth migration away from Tidy, we are deprecating the use of non-void self-closed HTML tags (so, to repeat, area, base, br, col, embed, hr, img, input, keygen, link, meta, param, source, track, wbr can be written in self-closed tag form and need not be changed). Additionally, we have started tagging pages using this invalid form with the [[Category:Pages using invalid self-closed HTML tags]] tracking category. Once pages which use this construct are cleaned up, we'll change both the "tidy" and the "no tidy" case to be consistent with the HTML5 parsing standard; that is, <b/> will be transformed into <b>.
Additionally, registered extension tags aren't subject to this consideration since they aren't HTML5 tags. So, for example, <ref /> and <references /> can continue to be used.