I use Firefox, which also has a responsive mode, but unless I'm missing something, the built-in tools don't load multiple copies of the website in different sizes side-by-side.
I have my own version of this tool (trivial javascript + iframes), and I use it frequently for one-glance CSS changes.
I have my own version of this tool (trivial javascript + iframes), and I use it frequently for one-glance CSS changes.