Avoid making pages that rely on browser plugins
Why does this matter?
Search engines often can't index content that requires browser plugins such as Java applets or Flash animations. Content that requires plugins will not show up in search results. Also, most mobile devices don't support plugins, and this can create frustrating experiences for mobile users.
Lighthouse displays the following failed audit if your content relies on plugins: "Document uses plugins".
Don't rely on browser plugins to display your content
To replace content that relies on plugins, you will need to refer to specific documentation for the plugin. For example, how to switch Flash video to HTML5 video.
Run the Lighthouse SEO Audit (Lighthouse > Options > SEO) and look for the results of the audit "Document uses plugins".