You can provide feedback on how to design an interactive web application that allows users to customize their owners with specific features of the interface. Your assignment is to create the code and CSS required to implement this feature. You should use only one line per message, add at least 5 extra constraints, utilize JavaScript libraries, and ensure compatibility across different operating systems like Windows, MacOS, Linux, and include a section on how you can achieve cross-platform consistency with detailed explanations for each step. Include an error handling mechanism that is robust to prevent crashes or failures during user interactions. Also provide unit tests for your code, along with performance considerations for high load scenarios.