Learning a new framework is no doubt one more step to do when it comes to creating a web page using only HTML and CSS. Why do we do that? In fact, taking extra steps learning any framework or even learning the usage of plug-in for editor languages would take up enough time and energy that would make anyone question if it is worth it.
Instead of going on and on about the pros and cons and personal preference on whether to learn an UI framework or not, it might be more suitable to narrow things down based on the goal to be achieved. What are you building? Are you building something very simple and it’s a one time thing that you won’t touch it again? Are you interested in building web pages and you might will stick with this hobby for a while? The thing is, HTML and CSS alone can be used to create good looking web pages, but when the web page you are building started to get more complicated on its framework as you constantly update and refine it, and as you have to start managing multiple complex web pages on your site, you might want a tool that could help you better manage the framework of your website without having to go through tedious formating to make your content display in a certain layout. How nice would it be if you can make your content displays in a framework you like with simply wrapping a couple lines of code around that content, so that when a UI framework would be appreciated.
Now, don’t stress on trying to make a decision of using a framework or not. You might be someone who happened to come across HTML because you have to use it for something else and probably won’t deal with it ever again, or you could be someone who constantly working on the building of web pages and wanted to collect all possible tools, just like collecting vocabularies when learning a language. Do whatever fits you!