According to the popularity of Android smartphone devices, the demand for Android application programmers has been increasing rapidly. Due to this fact, the subject of mobile application programming based on Android smartphones has become one of the important subjects in software development fields. In Android applications, the user interface takes an important role to make it interactive. Based on previous studies, the Android Programming Learning Assistance System (APLAS) has been used as a self-learning platform for Android programming studies. In this paper, we propose a web-based platform in APLAS for learning user interface design interactively using XML code. The 15 learning topics as learning materials cover the learning of UI layout and application resources. Students will get an assignment to be solved in each topic and the answer can be validated automatically on the server. To evaluate the effectiveness of this platform, we asked 40 students in an IT department in Indonesia to solve all topics. As the result, all students have successfully solved the 15 provided topics on schedule. Also, they gave positive comments regarding the ease of the learning process, which has confirmed the feasibility of our proposal to support learning UI design using XML.