Conversely, there are countless features introduced by Browser Vendors that do not exist in the W3C's specs. (And some of those are even eventually picked up by the W3c.)
I honestly don't think people understand how the Standards Body's work, and how their specs propagate into actual products that people use on a daily basis.
Many companies have cherry-picked what to implement and what not to. This is not new. If anything, it's the norm. Vendors will continue to do this as long as the W3C will exist, and can continue to do so in this case, without destroying the W3C or making it an invalid body.
This particular argument is totally bunk.
You wrote:
"I may be totally naive here, but I'm not really sure why this matters. That there is a WC3 standard does not imply that browsers have to adhere to it." - https://news.ycombinator.com/item?id=6491428
marcosdumay is responding to that by saying "It matters because ..."
And how many people will use it, apart from outliers like people who post here? The vast majority of people use one of the Big Three: Firefox, Internet Exploder [no, that's not a typo ;)], and Chrome.
Furthermore, if the Big Three implement a DRM standard, then web pages that want to "protect their content" will simply use the DRM standard, and it won't matter that Joe's Really Cool Browser doesn't implement it; that browser simply won't be able to view the pages. A few outliers like us will rant and rave; anyone else who tries it will say "Joe's Really Cool Browser Sucks" and go back to using one of the Big Three.
No, what he means is that, if we stop listening to the W3C because of this, the W3C will no longer matter. So either we won't have a standards body, or we will need a new one.