Added in rEOAT75c6f4eb9529: New UI for Special:AccountSecurity for T401774: Redesign UI for listing 2FA methods and adding new methods/T403683: Rename 2FA methods to friendlier names, the use of Html::successBox doesn't result in anything styled.. Should it?
It's fairly unnoticeable at the top as is.
Noticed while testing the patch for T404314: Redirect to Special:AccountSecurity once a new authentication method is added which reuses the same code, and displays similarly... not very noticeably.
<div class="cdx-message--success cdx-message cdx-message--block"><span class="cdx-message__icon"></span><div class="cdx-message__content"><strong>Authenticator app</strong> removed.</div></div>
