Title::isCssOrJsPage().
'TitleIsAlwaysKnown': Called when determining if a page exists. Allows
-overriding default behaviour for determining if a page exists. If $isKnown is
+overriding default behavior for determining if a page exists. If $isKnown is
kept as null, regular checks happen. If it's a boolean, this value is returned
by the isKnown method.
$title: Title object that is being checked