Caused by one of the changes going out this week, and hide-if was recently updated for OOUI but the signup form is still non-OOUI so presumably caused by 8910707.

Also, help text has disappeared. Enwiki has "help me choose" after the username, mediawiki.org hasn't. The extra whitespace is there so I guess HTML is getting stripped?

In T143850#2581254, @Tgr wrote:

Also, help text has disappeared. Enwiki has "help me choose" after the username, mediawiki.org hasn't. The extra whitespace is there so I guess HTML is getting stripped?

This is not a bug. This message seems to be intentionally disabled on mw.org – https://www.mediawiki.org/wiki/MediaWiki:Createacct-helpusername exists and is just   (it should probably be empty to avoid the extra whitespace, like on testwiki: https://test.wikipedia.org/wiki/MediaWiki:Wikimedia-createacct-helpusername).

(This message is overridden in the WikimediaMessages extension, and the magic there allows it to be customized under both of these names.)

Change 306616 had a related patch set uploaded (by Bartosz Dziewoński):
mw.htmlform: Don't refer to OO.ui if it might not be loaded

https://gerrit.wikimedia.org/r/306616

In T143850#2581256, @matmarex wrote:

(it should probably be empty to avoid the extra whitespace, like on testwiki: https://test.wikipedia.org/wiki/MediaWiki:Wikimedia-createacct-helpusername).

Thanks, that fixed it. The positioning of the help message seems off, but that wasn't broken by this train:

Change 306616 merged by jenkins-bot:
mw.htmlform: Don't refer to OO.ui if it might not be loaded

https://gerrit.wikimedia.org/r/306616

Change 306706 had a related patch set uploaded (by Bartosz Dziewoński):
mw.htmlform: Don't refer to OO.ui if it might not be loaded

https://gerrit.wikimedia.org/r/306706

Change 306706 merged by jenkins-bot:
mw.htmlform: Don't refer to OO.ui if it might not be loaded

https://gerrit.wikimedia.org/r/306706

Mentioned in SAL [2016-08-25T18:29:42Z] <thcipriani@tin> Synchronized php-1.28.0-wmf.16/resources/src/mediawiki/htmlform/hide-if.js: SWAT: [[gerrit:306706|mw.htmlform: Do not refer to OO.ui if it might not be loaded (T143850)]] (duration: 00m 49s)

In T143850#2581285, @Tgr wrote:

The positioning of the help message seems off, but that wasn't broken by this train:

Filed T143923: "(help me choose)" link near the username field in the signup form looks out of place, I'll look into this too.