The process of developing a web application for creating and testing prototypes of user interfaces is described. The results of analysing the processes of developing and testing interfaces are given. Similar applications are analysed. Based on the analysis, the shortcomings of similar systems are identified and the requirements for the system being developed are formulated. Mathematical models are worked out for automated evaluation of the quality of a graphical user interface based on the harmony of colours, visibility of objects, fonts, alignment, balance, space between elements, compliance with proportions and unity of style. The functional modelling of the system is carried out. A database is developed for storing prototypes and their testing results. An interface for an automated system for designing and testing user interfaces is created. A methodology for automating testing of user interfaces is formulated and algorithms for automated testing of prototypes according to various criteria are created on its basis. The application is implemented in Java and Java Script languages. It allows one to: create interface prototypes for all types of screens; assess visual literacy and structure of interface prototypes; offer solutions to correct aesthetic and stylistic errors; plan tests and add test tasks; conduct A/B testing; determine the focus of attention and the user movement route; determine the average time to complete tasks; record video step by step during usability testing.