If you look closely at the UITextViewLegacy methods, you'd see some references to styles and web objects. internally all the way up to iOS 7, the system was using WebKit for rendering! you'd set your styles using ObjC, that would turn to CSS behind the scenes. >>>
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If you've done any WebKit work you know what this is: One of the WKUIDelegate's methods! 👌👌👌 (https://developer.apple.com/documentation/webkit/webuidelegate/1387807-webview)
Someone forgot to remove a debug print in the shipping binary! at some point this was a web app, running in a UIWebView... in fact Ken Kocienda and Richard Williamson mention this in their CHM interview https://www.youtube.com/watch?v=xImAMe32Itg