[erlehmann_]

The parent poster is most likely using a strategy to build web sites called “graceful degradation”, where one builds “fallback” code paths for specific scenarios. The result is something that you see on web sites that have a “desktop” (all features) and a “mobile” (not all features) version.

What the parent poster wrote about testing is only true as long as one chooses “graceful degradation” as the strategy to build web sites. Using the alternative strategy “progressive enhancement” means that one does not have to test as much. The reason is that with “progressive enhancement” the more complex functionality of a web site is built on top of the simpler (think “no JS”) layers.

https://en.wikipedia.org/wiki/Graceful_degradation

https://en.wikipedia.org/wiki/Progressive_enhancement

It may seem counterintuitive, but choosing the right approach to build a web site can save a lot of testing effort – and even make sure that a site displays on browsers one did not even consider.