>Did any browser ever reject a Web page just because it wasn't well-formed XHTML?
Of course. If you served XHTML properly (by setting "application/xhtml+xml" MIME type), ill-formed XHTML would just show you a big syntax error instead of the page. Try it, that's still the case.
Even when being well-formed, lots of sites still used "text/html" type to trigger HTML (SGML) parser instead of XML one, as any 3rd party code embedded into the website would of course crash the page as well.
That was one of the reasons why XHTML never got popular and eventually has been abandoned.
That still wasn't serious enough. All it took to get browsers to accept non-XHTML pages was to change the MIME type. What I'm talking about is simply not displaying ill-formed pages at all, under any circumstances.
But with that MIME type it wasn't XHTML at all. It was being parsed as HTML which was possible only because of big similarity between those two formats. All you need to ensure the behavior you want is to disable HTML parsing (which is pretty much ensures being liberal in what the parser accepts already in its specification).
And this is precisely the point. If the very earliest browsers had insisted on correctness rather than permissively accepting broken HTML (and JS - see semicolon insertion), we would not now have a situation where browsers need to do a ton of work to allow graceful degradation in the face of awful markup, simply because the tooling would have evolved in the other direction. Postel's Law gains a little temporary sender convenience in exchange for a nasty mess of permanent receiver headaches.
Postel's law has two parts. Be gracious in what you receive is what most people are talking about, but be cautious in what you send is just as important. If people are using broken markup that's the problem that needs fixing. How many web devs bother with validation anymore? (And not for app like functionality, but for what should be simple text and images with a few menus - why are so many newspaper websites so awful?)
2.10. Robustness Principle
TCP implementations will follow a general principle of robustness: be
conservative in what you do, be liberal in what you accept from
others.
The problem is that being conservative and precise isn't enough. You and I can both be conservative, but disagree on the specifics (given an ambiguous spec, for instance). A permissive receiver of both our data now has to support both sides of our disagreement forever.
They do if they have any competition. Browsers are the perfect example here: a browser which responds to broken HTML by not working, but not exploding either, is going to lose out to one which does work. That means market forces pin the disagreement in place.
> but be cautious in what you send is just as important.
How are you going to enforce this for everyone?
> TCP implementations will follow a general principle of robustness (...)
This rule has worked well for TCP implementors in large part because of their circumstances, which are very different from those of browser implementors and Web developers:
(0) Priorities: How much do the following desiderata matter to each group: reliability, performance, new features?
(1) Skill: What skills does a representative programmer from each group have?
(2) Risk profile: How does each group cope with the possibility of design and implementation errors? How much technical debt are they willing to take?
I'd contend that Postel's law doesn't scale beyond relatively small groups of highly skilled programmers, for whom reliability is paramount and trumps all other considerations.
Exactly. So by being permissive from the start we now have this dumpster fire that prevents us from writing sane and performant code. Because we have to assume that with crap input the user doesn't want
Imagine what C++ would look like if all compilers had to accept all different variations of it, and the result of compiling 100 almost valid C++ files should, as far as possible, be a program that runs in some sense.
Most of the web pages I have ever written have probably been ill-formed because browsers don't tell me what's wrong, and instead show me a (nearly) working web page.
> Imagine what C++ would look like if all compilers had to accept all different variations of it, and the result of compiling 100 almost valid C++ files should, as far as possible, be a program that runs in some sense.
C++ is still a lot more permissive than it could and should be.
> Most of the web pages I have ever written have probably been ill-formed because browsers don't tell me what's wrong, and instead show me a (nearly) working web page.
Same here. The idea that JavaScript ought to be permissive and forgiving because its target audience doesn't know what they're doing turned out to be a self-fulfilling prophecy on the part of its designers.
It would be a lot more sensible than you think. When I get a compilation error, what I do is take a breath, think about the meaning of my code, correct any logical mistakes I can find, and try to compile again. Why couldn't Web developers do the same?
Also, there's no need to crash the tab. The browser could simply stop running any JavaScript, and leave the user with a static page.
IIRC some of the older versions of internet explorer did something like that, showing the user a popup if an error happened and asked them if they wanted to keep going.
Basically, give people a hand, and they'll grab your whole arm. It's human nature, and Web developers aren't above it.