100% this.
The moment I started to see HTML and CSS as a holistic system of layout construction, rather than as simply two separate but complimentary technologies, my productivity in both skyrocketed.
'Bugs' in my CSS became infinitely easier to not only find, but also solve. And the benefits of tools that both unified and abstracted layout construction, like Tailwind, became immediately obvious.
Shameless plug: I talk more about a lot of these mindset shifts and how they've helped me finally feel like I truly understand CSS in a book I'm working on called "How To Debug CSS" [1].
It goes over not only the mindset shifts that have helped me gain a decent level of mastery over CSS, but also shows you how to solve the 50+ most common issues that you'll come across when developing with it. Like when you have an overflow on your page for example, and have no idea what's causing it.