I thought I encountered a browser bug, but I didn’t. I learned some things, and now you can read this and learn them, too!
In a nutshell, progressive enhancement is building “content out”, separating the structure of a website (HTML) from its presentation and behavior (CSS and JS). Content should be available regardless of the capabilities of the device, and should not rely on styling or scripting to function at a base level. There are some strong opionions for and against, but it sounds like a good idea to me!