This is the trouble with web dev. There are a million factors in play, and it's very tough to create tests for them all, not to mention potentially financially restrictive with the need for multiple browser testing VMs. You have to test cross-browser, make sure all the versions of IE you want to support play nicely with your JavaScript and CSS. The site also needs to be responsive and handle resizing gracefully. Then how about making your site accessible, making sure all the aria tags are where they should be, and that screen readers will read, or not read, your content properly. When there is time to polish, it's often in the form of CSS transitions or animations.

Making my site fallback to no-js gracefully affects so few people, that it falls by the wayside since there's so many other higher priorities that affect a lot more people.