Originally, most on-line simulations were written using Java. Java is the programming language with the coffee cup logo. And Java programs were called applets. Unfortunately, serious security issues have cropped up, and most browsers and anti-virus programs now block Java programs no matter how hard you try.
Flash was a good alternative. But it too had security issues and is no longer available.Browsers today are much smarter than they were when Java and Flash were created. All modern browsers and most tablets & smart phones will run HTML5 & JavaScript (no relation to Java) simulations with no problems. Look for simulations tagged as HTML5 or with HTML 5 red shield logo.