That isn't really related to what he said at all. Sure web browsers should be forgiving in what they except, but dev tools shouldn't be. How would you like the C compiler that tried to interpret any old chicken scratch as a valid C program? Never a compiler error again!
Nope no analogy. If I expect a machine to understand a language, then it had better be able to determine weather or not a document written in that language is valid. It doesn't matter if it is a programming language or a mark up language. Permissive modes are great for accepting the work of others, but when learning how to write that language in the first place a strict interpretation is best. That way you can focus on getting it right.
C compilers do this all the time. If you're lucky, they'll warn you about undefined behavior as they do it, instead of just silently making all sorts of optimizations because the standard allows them to.
Now as it happens, C compilers have a syntax validator as part of them. Lots of HTML editors, past and present, have used HTML validators too...
Last I checked IE, Fire Fox and Chrome all had a dev mode. Sure they might not have had them at first but a strict mode or a parse check or some thing of that sort would have been helpful all along.
