What do we mean by “responsiveness”? Confusingly (and rather unhelpfully) when it comes to web applications there are two different topics that may be referred to by the terms “responsive” or “responsiveness”. If you stick “responsive UI” into your favourite search engine the top results will concern “responsive design” - the practice of making websites and applications work across devices, regardless of device and browser dimensions. That’s an interesting and important topic when it comes to designing data-science applications but it’s not what we’re covering here.