The explosion of the web happened in no small part because of how easy it was to write some HTML and get a basic, working webpage out of it. If you nested some tags the wrong way and the browser just put up an error page, rather than doing a (usually) pretty good job figuring out what you actually meant, people would get frustrated faster and not bother with it at all.
But imagine if our C/C++/Java/Rust/Go/etc. compilers were like "syntax error, but ehhhhh you probably meant to put a closing brace there, so let's just pretend you did". That would be a nightmare of bugs and security issues.
The difficulty in drawing a line in the sand and sticking to the spec, though, is that of user blame. Let's say you implement a spec perfectly -- even if you are the originator of the spec -- and then someone comes along and builds something of their own that writes out files that don't conform to the spec. Your software throws up an error and says "invalid file", but the other piece of software can read it back in just fine. Users don't know or care about specifications; they just know that your software "doesn't work" for the files they have, and the other software does. If you try to tell them that the file is bad, and the other software has a bug, they really won't care.