I think the authors of this document are missing the point of the site entirely, it's not about being 100% correct on all issues, it's about teaching people that have never seen code before what HTML JS and PHP is all about. They do that very very well - take it from someone that started from scratch primarily using W3Schools.
For people who use this site it doesn't much matter that all edge cases aren't covered, and that everything isn't entirely correct in the literal sense. What matters is that the content is accessible for Noobs. Here's an example from the linked Google doc: "If you declare a variable within a function, the variable can only be accessed within that function. When you exit the function, the variable is destroyed.
The whole concept of closures doesn’t exist at w3schools..."
when you're trying to figure out how to write your name on the screen 100 times using a loop closures aren't really that important... The general gist I get from it is that the people who wrote it don't really know what starting programming is like for normal people. It took me ages to figure out how to get javascript to execute in my HTML since all the documentation I could find was way above my level. W3Schools taught me. W3.org isn't a resource for beginners. "html5 Conformance Checker (alpha)", and "Game Development with JavaScript and the Canvas element" isn't really going to help people that are trying to make sense of the <P> tag.
W3Schools might be out of date, might have inaccuracies, and might not explain things fully, but it's exactly the right thing for absolute beginners.
Pease respect that.