I think it comes down to to a couple of things that software doesn't have that most other disciplines do:
Standardisation - in the big 'E' Engineering world, there would be a recognised international standard for Web Apps that ensured/enforced that all Web Apps supported this functionality, or they would not be approved for use.
Another factor is Accountability. A senior Software 'Engineer' would have to take personal responsibility (liability, accountability) that the software product they are producing and/or overseeing met all these requirements and personally sign off that these standards have been met. If the product were to fail at any point and it was determined that the cause was negligence in following the standard, any damages sought (not common, but not unheard of) would ultimately find their way to the accountable individual and their insurance.
In cases where budgets/importance don't allow for this level of scrutiny, there would still be paperwork signed by the producer of the software and the client acknowledging deviation from the standard and waiving any recourse for doing so.
there is totally standardization. At the building block level. TCP/IP, Protocols on top of that, language standards etc.
Web Apps are complex, why would there be a standard? Just like there's no standard for cars, other than for some of their components like wheels or headlights.
Aeroplanes are complex, but you can bet your life there are standards for those. And cars? Wow - I'm not sure which country you live in, but there are probably as many safety standards for road vehicles as there are for planes!
To continue the analogy from earlier - standards wouldn't mean all web applications would have to be designed, programmed and work exactly the same way, but it would mean that they would need to be formally tested (to an approved test plan), and to use your example, would need to demonstrate that each of those layers of fallbacks (as dictated by the standard and covered in the test plan) operate correctly in order to be certified.
If anything, I think software has a huge advantage over physical world engineering in that testing can be replicated at virtually no cost whenever a change is made to the design. I shudder to think how many cars get trashed in order to meet vehicle safety testing requirements.
There are thousands of these documents covering everything to do with transport from the vehicles to the reflectivity of street signs.
The regulation (at least in my state) is that only engineers who are registered as Registered Engineers are permitted to carry out professional engineering services in this state.
There are absolutely very well-defined and detailed standards for cars defined at both a national and international level. They range from excruciating requirements around single components (eg. ESC) through to broad design requirements (like how wide a car).
See the Australian Design Rules (which happens to form the basis of most UNECE and Canadian transport regulations) if you want to see how detailed they are.
Standardisation - in the big 'E' Engineering world, there would be a recognised international standard for Web Apps that ensured/enforced that all Web Apps supported this functionality, or they would not be approved for use.
Another factor is Accountability. A senior Software 'Engineer' would have to take personal responsibility (liability, accountability) that the software product they are producing and/or overseeing met all these requirements and personally sign off that these standards have been met. If the product were to fail at any point and it was determined that the cause was negligence in following the standard, any damages sought (not common, but not unheard of) would ultimately find their way to the accountable individual and their insurance.
In cases where budgets/importance don't allow for this level of scrutiny, there would still be paperwork signed by the producer of the software and the client acknowledging deviation from the standard and waiving any recourse for doing so.