In current societies, web application systems take central roles in computer systems. Thus, web client programming using JavaScript has increased values to add dynamic features and functions in web pages by well working with HTML and CSS. In this paper, as a new type of exercise problem for its self-study, we propose a code modification problem (CMP). In CMP, a source code with HTML/CSS elements and JavaScript functions for study, and the screenshots of both the original and the slightly altered web pages are provided to the students, who will need to edit the source code to generate the modified page. The goal of CMP is for students to carefully read the source code and comprehend how to use the components and functions through modifying parameters, values, or messages there. String matching is used to check the correctness of any answer. Through solving CMP instances, the students are expected to master the basic concepts of web client programming. To evaluate the proposal, we generated and assigned 25 CMP instances to 37 students in Okayama University. In addition, we offered project assignments of freely implementing source codes by referring to solved CMP instances to evaluate their learning effects. With the solution results, the validity of the proposal has been confirmed.