Web Platform Docs (WPD) is a site for documentation and education about Web standards, run by W3C and supported by Mozilla, among others. Anyone is welcome to contribute content from MDN to WebPlatform.org, under the terms of MDN's Creative Commons Share-Alike 2.5 (CC-BY-SA) license. Content that originates from MDN must be attributed to MDN, with a link to the source article. The Web Platform Docs site provides support for doing this in a systematic way.
If you create an account on WPD and create a new page, you’ll see that there is an option in the "new page" form to indicate that the content you’re contributing came from MDN, and to provide the original URL on MDN. There is also a way on WPD to mark sections of articles as coming from MDN, for cases where they get merged into CC-BY content. (CC-BY is the default license for reuse on WPD.) The External Attribution page on WebPlatform.org describes how to do all of that.