What is the "video/x-webm" MIME type

What is the video/x-webm MIME type? I know that there is the video/webm MIME type which refers to the WebM format but what about the x- prefix? What does it mean?

I wonder because I noticed that the current Chrome and Firefox versions unable to handle such type in the type attrbiute of the source tag:

<video controls>
  <source src="http://ift.tt/2bLJjo9" type="video/x-webm" />
</video>

http://ift.tt/2c20eX1

But if I change video/x-webm to video/webm it works as expected:

<video controls>
  <source src="http://ift.tt/2bLJjo9" type="video/webm" />
</video>

http://ift.tt/2bLJQ9L

Seems like I need to use the video/webm type but my OS returns video/x-webm when I pass such file to the MimeTypes from the Python's standard library:

from mimetypes import MimeTypes
import urllib
url = urllib.pathname2url('path/to/file.webm')
mime_type = MimeTypes().guess_type(url)
print(mime_type)  # ('video/x-webm', None)