Where did this idea came from?
In 2010 I had to develop an Intranet site for internal usage. I needed a lot of reading and review, which CMS should I use.
I began to use Joomla CMS. As a beginner PHP dev, I didn't understand a lot of things, but I made a custom template and it worked (wasn't beautiful, though). It caught my attention that by changing a few lines of code, you can change the entire layout of the page.
I began experimenting with CSS and HTML on my own, created a few templates just for fun.
...when Joomla is a bottleneck
Times change. I had to write and modify a lot of components for Joomla. Since Joomla is a very opinionated application, and I advanced in knowledge of PHP, I began working with CodeIgniter Framework. You can read more about this in app development link.
So I began developing an own CMS app with my own CSS framework. A good advice: never take a look at your old projects. You will regret, I guarantee. Mixing English and Hungarian class names, inconsistent name conventions, and a lot of things I don't want to talk about (silently cries in corner).
I made mistakes. Everyone does. But these mistakes are for learning from them. After a few months of CSS/HTML deep dive, I found a position at a private clinics as webdesigner. Full time webdesign, SEO and content management! Yay! Btw you can take a look at my portfolio, where you can find some projects from this era.
Responsive, mobile first
Mobile phones and tablets began gaining significant market share. I had to move towards them. Since I always hated the constant zooming, panning etc. when using a website without mobile support, I began specializing myself towards mobile first approach.
In the meantime, I stopped developing my own CSS framework, because Twitter Bootstrap basically did the same as my own stuff - with more components. I had no reason to continue my framework.
And what are your plans?
I began to drop Bootstrap from my projects, replacing with UIKit or Angular Material. Migrate some old projects to SPAs (where applicable). Learn .NET Core 2.0. And so on...