Multiple fixes: Libsass, a11y, silence warnings

After watching [this great video](https://www.youtube.com/watch?v=OOAsU9SUooA) about tab accessibility, I wanted to fix a few things, like the focus on the panel (instead of the first element) and add the aria-labelledby attribute ([See this part of the video](https://www.youtube.com/watch?v=OOAsU9SUooA#t=447))

Other fixes include Libsass compatibility and silencing in-module deprecation warnings.