The elemental technologies of WWW have been standardized as HTML and CSS, recommended by W3C. Therefore, not only the servers but also web browsers are considered as components independent within the whole web application system so that they should be interchangeable. However, the actual web application has not yet led to this ideal situation. The problem of interoperability discrepancies between different implementation of web browsers still remains. So far we had studied the reason why this problem arose and collected instances of problem. Based on the result of our previous study, we developed an web-system designers' tool which can provide the useful information for them to avoid the pitfalls on the interoperability discrepancy problems.