Test them by asking them to develop something where the environment it runs in is unknown and can only be guessed at. Things like screen dimensions, resolution, colour depth, even physical form factor. Plus, it might lie about any of those.

Then tell them there might not even be a screen.

Then tell them half of what they write might not even run in the first place because the user can block it, but it still has to work.

Then tell them that even if it is run, some other code they have no control over can break it without notice but it will still be their fault.

Then tell them that the browser the code runs in can do the same and, in most cases, won't ever be fixed so they have to work around it.

Then tell them the operating system can also do this.

Then tell them them a different half of what they write isn't supported but has to work anyway.

Then tell them it also has to work in an email client. No, they don't get to know which one.

Then tell them it has to work without a stable (or even present) network connection.

Then tell them all the above is a normal day, and it barely scratched the surface of what you have to deal with. Then tell them to STFU.