Yeah I can relate. Most designers rush through the html part to the CSS part because it's more lively than the former. I started Web design recently and it's the most interesting part for my.When it comes down to whatever you are programming, what's your favorite source of code that you like. What do you aim mostly at doing and why?
Hell, CSS is fun, if you like playing with colours LOL
Bootstrap can be included in many coding like CDN in Angular, Nodejs, EJS, ReactJS because it can be implemented in various payment gateway or shop websites or any devellopement site that requires mobile rendering.If you know Javascipt and CSS than I think Bootstrap is best coding option for you but CSS still best as you can twist your site appearance easily. Node.Js is also quite easy and favorite for JS.
CSS is frontend develloppment except if you are working with Unwind Tail CSS where you have to compile a code before be build into CSS code.The CSS programming language or code is actually my best source of coding as it is very simple and colourful to make use of it.