The important part on that page is: Working Draft. Microsoft has committed to supporting standards, as soon as they are standards. It's certainly arguable whether it's a good idea to implement draft standards that people write HTML for that then change under them.
Also, this kinda seems like cheating: "While only recently a W3C specification, this system has been in use for some time by Mozilla and Apple for interface purposes." So they codified something that Mozilla and Apple were already doing, then said that older versions of the browsers already supported the non-existent standard. Impressive.
"Working Draft. Microsoft has committed to supporting standards, as soon as they are standards."
As you can see in the example pixelcloud cites, Microsoft does in fact implement features based on working drafts. They are already doing this with CSS3 transforms and will be doing so with CSS3 gradients, animations, transitions and flexbox. Therefore, that doesn't actually appear to be the issue at all.
"It's certainly arguable whether it's a good idea to implement draft standards that people write HTML for that then change under them."
Well, we are discussing CSS here, and that's what vendor prefixes (which Microsoft uses, too) help avoid.
"So they codified something that Mozilla and Apple were already doing,"
This is basically how the CSS3 standard progresses. Browser vendors implement a vendor-prefixed feature, and that feature works its way through the standards process. In fact, Microsoft does the same: http://caniuse.com/css-grid
Also, this kinda seems like cheating: "While only recently a W3C specification, this system has been in use for some time by Mozilla and Apple for interface purposes." So they codified something that Mozilla and Apple were already doing, then said that older versions of the browsers already supported the non-existent standard. Impressive.