Progressive vs Graceful Degration
In Graceful degration developer develop software for more advance version of browser, but also ensure that it should function properly on older browser version.
So Progressive enhancement and Graceful degration altogether is a different concept. Progressive enhancement's main focus is on content, layer separation between structure, presentation and behavior, whereas Graceful degration's main focus is the browser.
So if your software supports newer version and as well as older version of browser then it means your software is following the Graceful degration methodology.
If your website's more focus is on content and has layer separation (i.e. structure, presentation and behavior) and your website support newer and older versions of browser then you are following both methodologies in your application.