>There's a real argument that in a hackathon 10% extra time isn't worth it, but in the real world where you don't always have 48 hour time limits I've always been in the opinion that the initial time investment is far outweighed by the benefits and the additional cost that you would incur by making it mobile-ready later (which usually requires either rebuilding the frontend templates from scratch or hacking away at the existing codebase in a way that is far less organized/sustainable).Oh, I completely agree with this. I'd say that even during hackathons, mobile is definitely something you should keep in mind when you're building your design, markup, etc. just like you normally would so that things will be easier in the long run. However, the key thing is that when you hit something that would be easy to deal with on desktop but hard on mobile, you should just go the easy desktop route and leave the mobile solution for later (post-hackathon). And when you're building web apps, those kinds of moments will pretty much inevitably come because mobile is hard - compared to desktop, you will generally speaking always have less screen real estate (and input elements need to be larger, making it even smaller), a lot less processing power, lack of precise pointer input and hover (touch vs mouse), etc etc.
Again, the point here is the difference between building websites and web apps. With websites, your biggest concerns are generally that the site scales nicely to various screen sizes and that links are comfortably clickable (and that it all looks nice). This wouldn't be all that hard to do even under time constraints. Web apps on the other hand generally feature a lot more (single-page) interactivity, and getting that right is a lot harder.