Progressive enhancement focuses first on content, which is the purpose of all informational technology. Atop content sits presentation, and beyond that is client-side behavior.rG3 Each layer adds more evolving technology, but the lower layer is more stable across platforms.
This approach contrasts graceful degradation which typically refines in the opposite direction.
The content-first approach is fundamental to mobile-first design.
Aaron Gustafson, “Understanding Progressive Enhancement,” A List Apart, October 7, 2008, https://alistapart.com/article/understandingprogressiveenhancement/. (See notes.)