https://kapeli.com/mdn_offline
https://developer.mozilla.org/en-US/docs/MDN/About#Downloading_content
https://stackoverflow.com/questions/9501882/mdn-javascript-docs-for-offline-use
Download it at https://mdn-downloads.s3-us-west-2.amazonaws.com/developer.mozilla.org.tar.gz
The "Downloading content" section of About MDN provides the above link (for a tarball download)
along with guidance on other ways to access the MDN content, both as single pages and via third-party tools.
And others : don't mirror with wget & co, this is putting un-needed pressure on the website and hinders other users.
At least make sure https://developer.mozilla.org/robots.txt (which asks for gentle throttling) is properly handled.
Wget does not handle this for instance (http://savannah.gnu.org/bugs/?30999).
You can download the MDN docs using the table below. Before downloading, please consider using a
documentation browsing app like Dash (macOS), Velocity (Windows) or Zeal (Linux) instead.
These apps will help you get the most out of the docs, by providing a search index and quick access to everything you need.