An application is considered "usable" if it is visually appealing, simple to use, and performs as expected. The majority of IT industries focus on the app's functional requirements while making only minor efforts to improve usability. Usability is an essential attribute that want more attention in determining the production of a successful mobile application. In recent years, different learning applications have been developed to help the effective learning and enhanced the IT expertise of the students but the usability of apps for slow learners is ignored during design process so that the slow learners are facing problem during learning process. In order for slow learners to learn effectively, learning apps must focus on usability so that they can learn effectively as learn others. Slow learners are students who have difficulty in learning due of their limited cognitive abilities. They face difficulties in understanding complex instructions and take longer to catch up. Smart phones can be used as complementary learning tools to support and enhance the learning experience of the slow learners. The touch screens to interact the applications on the smart phone gives the more confidence of slow learners to feel more encouraged to stay maximum period and to involve with the application. Using apps in learning activities with slow learners reaps benefits for their learning. In this work, we performed a participant-based usability evaluation of the 25 apps on HTML learning for iOS and Android. Slow learners between the ages of 16 to 55 years are selected for usability evaluation. Further participants are distributed into four main groups Group I, Group II, Group III and Group IV based on their age and 24 participants/slow learners are selected for usability testing under the supervision of moderator. As a result of usability testing of HTML learning apps, we discovered that HTML learning apps usability decrease as the age of the slow learners participants increase. Current HTML learning apps are not effective and efficient for slow learners learning.