I built an app in Excel 5 years ago. It was a heat map for events, for stadiums, theaters, etc. I would build the static CSS/HTML and write out the files from Excel. It is still in use today. After having stopped working on it for a couple of years a minor bug was brought up to me recently. Luckily I had a windows computer again with Excel installed. Excel is a great platform for building prototype apps, or even long lived apps - especially 10 years ago. It is amazing the power that Excel has. But, a proper app should be built in JavaScript with a solid back end (F# being my fave). Although developing in Excel was a bit clunky it was amazingly easy to get beautiful customer interfaces and beautiful forms.