At least IMHO they are not equivalent. Why not ?
Conversely the developer writing the functionality might never touch a single line of HTML or CSS code to make the layout look any particular way. Layout and design are not necessarily their role.
Now there are even people who understand all these technologies that are used in the browser and all the back end technologies used to implement the additional functionality of a web site. Usually they’re known as “full stack developers“.
A person like this has learned the language of web designers, generally html and css, some programming languages, php and javscript are common, more than likely some web based framework for various controls and layout like jQuery or Angular, and usually has some knowledge about databases like mySQL or postgresql for the back end.
EDIT : 12:47 I’ve received a comment offline where a confused reader seems to think I implied that one is more important than the other. I make no claim one way or the other and, in software, they both play an important but different role. An ugly app is an ugly app. And one that doesn’t do what it says just doesn’t and so both are required whether its a successful app, web app, or web site.