I started looking at CSS Frameworks quite late in the game. It seems that most of the "headaches" that different browsers presented Web designer with in the paat have largely been overcome.
Although I have been playing around with web design for over 15 years a lot has been developed over that time. If you read the WikiP page on the history of CSS you will see some of the "tales of woe" that I was looking at earlier. Even the quote below relating to Frameworks is from a book published in 2012. W3C now have their own framework and also have a design reference to Bootstrap.
Both Bootstrap and W3.css are Frameworks. W3.css is "published" and you can download it and run it from your own server. As such you cn also edit it but I can't see much point in that.
Bootstrap is also open source ad can be downloaded. Wordpress also uses Bootstrap and hence the comparison below. I have made my views on the use of Wordpress clear.
What the WikiP page on CSS says about Frameworks
" CSS frameworks are pre-prepared libraries that are meant to allow for easier, more standards-compliant styling of web pages using the Cascading Style Sheets language. CSS frameworks include Blueprint, Bootstrap, Cascade Framework, Foundation and Materialize. Like programming and scripting language libraries, CSS frameworks are usually incorporated as external .css sheets referenced in the HTML <head>. They provide a number of ready-made options for designing and laying out the web page. Although many of these frameworks have been published, some authors use them mostly for rapid prototyping, or for learning from, and prefer to 'handcraft' CSS that is appropriate to each published site without the design, maintenance and download overhead of having many unused features in the site's styling. "