The MediaError.message DOMString field provides, if available, any additional vendor-specific error message detail to assist web authors debugging media player errors. The format and content of the string is left to vendors, though it's commonly in the form of a vendor-specific error code followed optionally by a ":" and further detail about the error. It is populated at the time of MediaError creation.
Docs: https://html.spec.whatwg.org/multipage/embedded-content.html#dom-mediaerror-message https://developer.mozilla.org/en-US/docs/Web/API/MediaError/message
Samples: https://googlechrome.github.io/samples/media/error-message.html