This is a simple concept of an off canvas, responsive CSS3 menu. It uses purely CSS, albeit some pretty modern features (transforms, flexbox) that aren’t compatible with older browsers.
This example uses an interesting technique with hidden checkboxes, labels, and adjacent HTML elements. The checkbox toggle method is an interesting method for toggling elements without using jQuery. You can read a more in-depth description of the method from my older post on a CSS-only multi-step form.