When I was coming up as a young web developer. the concept of progressive enhancement was kind of at the hipster vanguard of web development. This was in the phase of the web where JavaScript and CSS feature parity amongst browsers could not be taken for granted like it is today.
The idea of progressive enhancement was that you started by making your web application work on an essential informational level with semantic markup, then you layered on presentational and dynamic niceties with CSS and JavaScript respectively, if available to the user agent. At each layer, or each stage, you left the user with something usable and valuable that stood on its own without the other stuff.
Learning about the concept of progressive enhancement early in my career colored my interpretation of the Agile philosophy forever. The iterative nature of Agile methodologies, with "Working software [as] the primary measure of progress" is essential to how I approach making software for users to this day. In my mind's eye, I still visualize iteration as progressive enhancement. We start with the essential core of value, and ship that. If the core of the idea resonates with real users, then we iteratively layer on enhancements to the core, shipping to users at each stage.
In order to work this way, you have to accept that quality and feature completeness are not the same thing. At each layer, we're delivering a scope with high quality always, but with no ambition of completeness. We're delivering shippable increments of software as layers over a core idea of value that we've already shipped. We build out our product backlog as all the nice things we could layer over the core, as we think of them, but they are not essential. One of the amazing things about working this way is that users are delighted by it. The features just keep getting nicer before their eyes.
