High Performance Images

Chapter 11. Responsive Images

Yoav Weiss

There’s no doubt that Responsive Web Design (RWD) has had a huge impact on the way we build websites nowadays. If you are operating a website, chances are that it’s either responsive already, or it will be soon enough. There is no other way to serve websites while coping with the diversity of devices your users are using to access your site, and providing these users a pleasant experience.

But it’s not all sunshine and rainbows. The issue of responsive images has been a thorn in RWD’s side for a long while and a huge source of pain for developers trying to implement performant responsive websites.

How RWD Started

Early on, when RWD was coined in Ethan Marcotte’s seminal article, the approach to images was fairly simple, if not to say naive: just send the browser the largest possible image and let it resize it on the client side to match the responsive layout.

While that approach works when testing the simpler use cases over the local network or even on a high-speed broadband network, it fails miserably when we take into account the reason RWD was needed in the first place: mobile devices over cellular networks. With the earlier approach we’re sending unnecessarily large images to mobile devices, often over poor connectivity.

The immediate result of that approach was that RWD got a reputation for being slow, and it became obvious to many that a responsive website meant a bloated one.

The web developer community realized that RWD is the ...