A very easy way to ensure a website works even if not all your assumptions are met (e.g. CSS, JS, fonts are loaded properly) is called “progressive enhancement”

A very easy way to ensure a website works even if not all your assumptions are met (e.g. CSS, JS, fonts are loaded properly) is called “progressive enhancement”: Build the system as good as you can without JS, then add JS on top for the things you can only do with JS. All web sites I created using that technique required minimal testing on systems without JS. https://en.wikipedia.org/wiki/Progressive_enhancement https://en.wikipedia.org/wiki/Progressive_enhancement