The progressive enhancement is as they say in the article that the coding involve in Web building is like the peanut M&M is built of the peanut center (html) the chocolate coating (CSS) and the candy outer shell (Javascript). The only part that is effected by the progressive enhancement is the CSS and Javascript. The progress of building websites slowly and carefully by piece by peice putting it together starting with html core site. After getting all the html to work properly you can start the beautification of the site with CSS. After applying the fine coat of paint on the site now is time for the moving pieces of the site that makes people go “wow”.
Related Link : https://www.webdesignerdepot.com/2010/08/a-complete-guide-to-progressive-enhancement/
Trevor's Coding Talk 