Lighthouse flags pages that use plugins:
Lighthouse checks the page for elements that commonly represent plugins:
Lighthouse then flags an element as a plugin if its MIME type matches any of the following:
Lighthouse also flags elements that point to a URL with a file format that is known to represent plugin 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.