There's nothing idiotic about writing code and looking at it in a browser, and seeing it work, and therefore assuming you wrote working code.
Conversely, if it doesn't work, you'd assume you didn't write correct code. The conditions for what is correct and incorrect code are arbitary enough that it doesn't really matter what they are, as long as it is made clear when and when not the code is correct.
In summary: If browsers don't display invalid code correctly, people will write valid code. If browsers try their best to display invalid code, people will write invalid code.