PendingTextInputWidget is used by TagMultiselectWidget and its children to display a widget in the loading state before the widget can be infused in JavaScript. However, if JavaScript is disabled or fails to load, the pending widget remains there, see screenshot:
The textarea is meant to be used as the input element if JS is not available, hence I believe the pending element should simply be hidden if JavaScript is not available (client-nojs).
(This task would need a mediawiki-widgets tag that doesn't exist. I hope HTMLForm and OOUI are close enough to let potentially interested people see this task)