I think that HTML, CSS and JavaScript (ES6) could be considered to be the foundations upon which the various other technologies are built. I think it is import

I think that HTML, CSS and JavaScript (ES6) could be considered to be the foundations upon which the various other technologies are built. I think it is important to understand the boundaries between these areas and how they interact. In my experience you will probably need to understand the trade-offs between using JS or CSS3 for certain effects. Another key understanding is to really understand the boundary between JS and DOM. JS (as an implementation of ECMAScript) treats DOM as a collection of external objects. And of course, those DOM objects do have variations between browsers and their interactions with CSS. I think it helps to understand JS as a programming language, e.g. in a NodeJS or Duktape environment. IMHO PHP is less relevant in a 2018 context. There are many frameworks out there (I'm no expert on them) which provide far more useful abstractions for building WebApps, etc.