It's a fun little soundbite to talk about how the web is responsive right out of the box. With no authored CSS at all, a website will flow to whatever screen width is available. If your site isn't responsive, you broke it.
Well that's almost true, but as Adam Morse says in this new project:
HTML is almost 100% responsive out of the box. These 115 bytes of css fix the 'almost' part.
Things like images and tables can have a set widths that would force a layout wider than a viewport. And of course, the meta tag.