HTML plug-ins are used to extend the functionality of web browsers, allowing them to display or interact with content that is not natively supported by the browser. Some examples of popular plug-ins include Adobe Flash Player, Microsoft Silverlight, and Java Runtime Environment.
Plug-ins are typically written using programming languages such as C++ or Java, and must be installed on the user’s system in order to function properly. Once installed, they can be invoked by web pages using HTML or JavaScript.
However, plug-ins have become less common in recent years, as many web technologies have evolved to provide similar or better functionality using standard web technologies such as HTML5, CSS, and JavaScript. This shift away from plug-ins has been driven in part by concerns about security vulnerabilities and the need for better cross-platform compatibility.