As mentioned, prior to the introduction of HTML5, the only way to add media such as audio and video to your Web page was through a third-party plugin.
These third-party plugins can be a bit of a black box, and you’re relying on the user to actually have them installed. With something like Flash Player, it was quite likely the user would have it installed, because it has a wide market share and is installed on a lot of user systems.
But there were problems. The code for embedding a Flash video in one browser didn’t necessarily work in another, and there weren’t any elegant ways around it. Plus, there was always a speed issue, because the browser hands off the playing of the media content to the plugin.