Search engines often can't index content that relies on browser plugins, such as Java or Flash. That means plugin-based content doesn't show up in search results.
Also, most mobile devices don't support plugins, which creates frustrating experiences for mobile users.
How the Lighthouse plugins audit fails
Lighthouse flags pages that use plugins:
Lighthouse checks the page for elements that commonly represent plugins:
embedobjectapplet
Lighthouse then flags an element as a plugin if its MIME type matches any of the following:
application/x-java-appletapplication/x-java-beanapplication/x-shockwave-flashapplication/x-silverlightapplication/x-silverlight-2
Lighthouse also flags elements that point to a URL with a file format that is known to represent plugin content:
swfflvclassxap
Don't use plugins to display your content
To convert plugin-based content to HTML, refer to guidance for that plugin. For example, MDN explains how to convert Flash video to HTML5 video.