I have a debug tab in my Lovelace UI that I use to play around with different features and new entities or card types. I think it would be very useful to have a page that works similar to the “Template” Developer Tool but with Lovelace cards instead of templates. Basically, one panel with yaml input and another displays what that would render as if it were a tab in your ui-lovelace file. This is also useful for pulling up data, for instance, if I want to compare the history of two entities I can quickly make a history graph of those two.