It seems there's unused code for banner previews in an iframe as part of the banner editor. We should remove this.
Can this task be reversed to actually add a banner preview while editing?
When creating banners I am forced to use such handmade makeshift preview: https://meta.wikimedia.org/wiki/User:Base/CNBannerEditpreview.js and of course it does not cover everything I need and still forces me to save the edit (which as per another bug is slow), view live preview, and repeat it many times, sometimes to do something as trivial as getting the font-size right.
I have no concerns if it is easier to remove whatever legacy unused code and just build a new previewer from scratch in the future though