Migrating away from web plugin technologies
Historically, web plugins such as Adobe Flash (and Flex), Java applets, Microsoft Silverlight, Apple Quicktime etc were used to deliver rich user experience to users in HTML-only browsers. This included everything from music and video players, through games to full applications with charts, graphics and rich user interface.
These however introduced a number of problems, primarily related to compatibility and security. Plugin-based applications required the user to install the plugin for a specific operating system and web browser, and upgrade it along with the browser and system upgrades. The backwards compatibility legacy also negatively impacted security, especially in case of Adobe Flash, which in late 2000's suffered from recurring waves of critical security vulnerabilities.
As HTML5 support became more widespread offering the same experience but in standards-based manner, web browser vendors started to gradually announce end-of-life for these plugins and invest into supporting specific features through HTML5 implementations.
In long-term website owners should thus plan to replace the plugin-based functionality with one based on HTML5 features. For example, Adobe Flash is scheduled for end-of-life in 2020 and Microsoft Silverlight in 2021.
Further reading: StackOverflow Flash is Dead: What Technologies Might Be Next?