Rachel Andrew, once again, seriously hits the nail right on the head:
There is something remarkable about the fact that, with everything we have created in the past 20 years or so, I can still take a complete beginner and teach them to build a simple webpage with HTML and CSS, in a day. We don’t need to talk about tools or frameworks, learn how to make a pull request or drag vast amounts of code onto our computer via npm to make that start. We just need a text editor and a few hours. This is how we make things show up on a webpage.
I’ve been building interfaces for the web since 1996. I first picked this up when I found out that Netscape came with something called Netscape Composer. It didn’t take me long to realise that there was something beneath that WYSIWYG-like editor that interested me even more; HTML. Fast forwards a few years and I had now found out about the wonders of CSS (and the horrors of the browser wars of the early 2000s). One thing have remained constant right up until today; the web still consists of HTML and CSS. No matter the tooling, no matter the frameworks, you still end up with these things (and JavaScript, of course).
Learning the basics is not only a great entry point, as Rachel points out, it’s also a vital skill if you want to become a truly great web developer.