The subtle difference between progressive enhancement and graceful degradation is lost on too many. Many of those that support “mobile first” don’t support progressive enhancement, which is an oxymoron.
Mobile first is progressive enhancement. Start with the most important part of any site: content. Mark it up. Semantically. Why? Because some readers experience your site without CSS. It’s true. Some just want the content. Screenreaders and search engines are the largest of these.
Then style the content. That’s the CSS. Mobile first says start with the LCD, the Lowest Common Denominator: mobile. But make sure that your CSS works in the LCD. That might be IE 9. It might be IE 11. Look at your analytics if possible. Progressive enhancement means add features when available. Progressively enhancing. You see? So if you are using a feature that is only available to IE 11 and better (Chrome, Firefox, etc.) and you need to support IE 9, make sure that the feature is not necessary for the user experience.
We need to get back to progressive enhancement as the standard. We need to get back to the promise that the web is accessible to all, no matter the device they use.