FitVids.js is a lightweight jQuery plugin created by some very talented people: Chris Coyier & the guys at Paravel. Simply enough, it makes your video embeds (i.e. YouTube & Vimeo) responsive. To use it, you just select the container element of your videos (more in the instructions) and FitVids does its magic. You can install the “FitVids for WordPress” plugin or you can install the actual jQuery script yourself by downloading it from GitHub (Chris has an awesome tutorial for installing FitVids.js on WordPress).