CSS Frameworks
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 past 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.
I started to look at Bootstrap when I was looking at the design of the Wikipedia pages. See my page on Bootstrap Dynamic Tabs.
What the WikiP page on CSS says about Frameworks