The signup form (Special:CreateAccount) uses responsive CSS to simplify the form on small screens. This is tailored to the configuration used on standard Wikimedia wikis during normal signup and breaks otherwise:
- the real name field (on wiki where $wgHiddenPrefs does not include realname) gets hidden entirely
- the creation reason (when visiting e.g. a Wikimedia wiki's signup page while logged in) gets hidden entirely
- custom fields like the shell username on Wikitech (https://wikitech.wikimedia.org/w/index.php?title=Special:CreateAccount&mobileaction=toggle_view_mobile) are shown without any label or other indication of what the field is about
Hiding form labels and leaving placeholders is a bad accessibility practice: placeholders can be hard to remember, and people can fill out information in steps, especially on registration forms, and in case where they have filled in something already, they cannot see what field it was in a form that does not have labels at the top of the form. Modern Codex inputs do not feature this ‘feature’, and neither did OOUI inputs, and this seems like a holdover from a MediaWiki UI era that can be safely removed without much problem.
On mobile (with smartphone media query, not tablet) the placeholders take a more important role, as they are the only field indicators.
That's questionable on it's own, as the information, what field the user is currently typing in, is disappearing after the first character is typed.