The `vw` unit is unrelated to responsive images but a new unit type that reflects the Viewport Width (100vw = 100% of browser’s viewport). There’s also a not so good supported vh unit, all specified under CSS Values and Units Module Level 3 (http://www.w3.org/TR/css3-values/).

The `calc()` function is a CSS function that lets you calculate values like 100vw - 100px, similar to what you can do in JavaScript. See more: https://developer.mozilla.org/en-US/docs/Web/CSS/calc