Runescape was made with Java as an applet game. I believe it uses JOGL, which is the Java wrapper for OpenGL.
It would basically be impossible for one person to create it from scratch. But if someone did have
tons of experience with Java and JOGL, it would likely take them about 5 years or more to create the entire server, applet and all the content. While Runescape was originally created by two brothers, the original version was nothing compared to what it is today. The version today has been developed over several years by a fully-staffed, large development company.
A more realistic goal is using an exiting game engine that can run in a web browser such as
Unity 3D.