Which CSS framework is popular among elixir? I am new to phoenix and elixir. Which framework is good and adhere the principles of Pheonix?
Phoenix is not really opinionated about which CSS framework you use.
At any rate, my favorite CSS framework is by far Tailwind and I highly recommend it if you are comfortable with utility/atomic styling. It is a lower level framework that can be used to build your own custom design systems. This is going to be different than the batteries-included options like Bootstrap and Bulma.
Any CSS framework will work fine. I personally like BEM, which is not really a framework. http://getbem.com