Consider the following use case:

An extension (e.g. InputBox) creates input fields, if a special parser tag is added in wikitext. Currently OOJs UI in core can be easily enabled with OutputPage::enableOOUI(). For these extensions it isn't easily possible to add OOUI elements in the parser function. As a workaround they could enableOOUI via:

RequestContext::getMain()->getOutput()->enableOOUI();

which would workaround a fatal error, which would throw, if you use any OOUI element without setting a theme.

But if a user views a page, the form will be unstyled, because the oojs-ui Resource modules aren't added to OutputPage when the page is viewed. This could be fixed by another workaround, by using the OutputPageParserOutput hook:

public static function onOutputPageParserOutput( OutputPage &$out, ParserOutput $parserOutput ) {
	if ( $parserOutput->getProperty( 'enableOOUI' ) ) {
		$out->enableOOUI();
	}
}

Both workarounds work very well, but it feels a bit bad to let extensions handle this anytime again. There should be a better approach, e.g.:

• enableOOUI() in ParserOutput, too, which enables OOUI
• enableOOUI() set's the 'enableOOUI' property by itself (or any other reasonable property)
• if enableOOUI() is set, OutputPage should enable OOUI by itself, when the ParserOutput object is added to it