Of course Bootstrap is not the only framework out there and some people might not like it. I think it's fine, I've been using it for a few years and see no problems with it thus far. For the sake of context, let's take a look at similiar frontend frameworks. Also, what does "frontend" even mean?
In software development, you could split the web development area into "frontend" and "backend" layers. Frontend stands for the code that takes care of the "front" of the app, or "the side facing the user".
Apps can function perfectly fine without a frontend layer, they communicate via network in a similiar way that you request webpages, by navigating via links, but they don't need a browser or any visual elements to do it.
Websites can also function without any "backend" code. For example, any social media website - You can register, login, post, send messages and so on. Once you press "login", fill out the form and send out a request to login, the frontend layer only exists to provide you a fancy form to fill out (and sometimes to make sure you're not typing wrong stuff).
The form notifies the server that is hosting the webpage, with the info you entered, then the server does all of the heavy lifting of finding you in the database, verifying that your password is the same as the last saved one in the database. Then if succesful it sends you to your dashboard, loads up your "wall" and etc.
If a website doesn't have any of that, that's ok. All it really needs is just the .html document. So effectively you could host your own webpage right now. All that's left would be to find a hosting company, get a subscription, buy a domain in a domain registrar and moving the files from your computer to the folder on the server. That's it!
So what are the top 10 CSS frameworks out there right now?
We already looked at Bootstrap, so let's go to #2.
Foundation boasts itself as "The most advanced responsive front-end framework in the world." and I honestly cannot argue, but mostly because I haven't used it as much. I love that they have E-mail templates too, because I have tried myself to build an e-mail template once and it was not pretty.
I really like their approach of having all elements on the same page too. Very convenient, in my opinion.
As you see it is similiar to Bootstrap (you might even start to recognize websites using these libraries too), but their approach to the grid is different.
I really like UIkit. The first thing that stood out to me is the design style of the components, to me it seems that the default one is the most universal of the three so far. It's sleek and modern.
You'll see a lot of the same component ideas, but same as last time, their grid approach is different too.
Since I haven't used Semantic UI, Bulma, Tailwind and others, I will avoid providing an opinion about those. My suggestion is to try them out to find the one that you feel most comfortable with and only with practice you will see the pros and cons of each.