HTTP Strict Transport Security (HSTS) informs browsers that the site should only be accessed using HTTPS.
Modernizr is a JavaScript library that detects the features available in a user's browser.
PayPal is an online payments system that supports online money transfers and serves as an electronic alternative to traditional paper methods like checks and money orders.
Twitter is a 'microblogging' system that allows you to send and receive short posts called tweets.
YouTube is a video sharing service where users can create their own profile, upload videos, watch, like and comment on other videos.