I think this point needs to be stressed more. It is dead simple to write a website that works well on mobile phones. In fact, the first ever website, without CSS, without any JS, without fancy HTML5 features, is mobile friendly: info.cern.ch/hypertext/WWW/TheProject.html
It’s only when you start adding useless bells and whistles like floating shit in from left and right, tons of animations, side-by-side displays, overlays and whatnot that you need to start being competent to make it work on mobile.