Progressive enhancement introduced visually with OpenType font features: basic at first, then ligatures, alternate glyph variants, swashes, …
But what is that—progressive enhancement? It “isn’t a technology. It’s more like a way of thinking,” says Jeremy Keith.
The key to thinking this way is: accepting that websites/web applications do not need to look/behave the same in every browser. Make the core functionality available using the simplest technology, then enhance from there—secure in the knowledge that even if a certain feature is not supported in some browsers or on some devices, that’s okay: the fallback is in place and works everywhere.