Migrate has to make some shaky assumptions about the implementation of core's animations in order to warn about jQuery.fx.interval. It would be simpler if we used a private value for the interval value (basically just hard-code the default 13) so that any use of jQuery.fx.interval could be indentified as deprecated. There is a slight risk that someone might be changing that value but it seems unlikely since we are already ignoring it except for two rare-ish cases: 1) IE9 always, 2) browsers when the document is hidden.

In addition, per some discussion on jquery/jquery-migrate#298 we may want to change from document.hidden to document.visibilityState since the standard has changed. From looking at caniuse.com it seems like IE10/IE11 support rAF and document.hidden but not document.visibilityState so I am not sure that is possible until we drop their support.