The Web is increasingly mobile; people are increasingly using phones and tablets, in addition to desktop computers.  We hear a great deal about how we thus need to take mobile devices into account in our development work — but “mobile” means much more than having a responsive site. We need to take mobile devices’ speeds, battery lives, UI, and sizes into account when considering our Web application performance. In this talk, Estelle Weyl tells us what Web developers need to know in order to develop sites that are high quality for mobile devices, as well as for the desktop.