"Fixing patterns instead of sites" I don't understand this statement
They aren't targeting specific sites that are broken they are targeting more general patterns that seem to cause problems.
If you write html/css in a relatively compliant way, things will not be so broken that a browser vendor needs to account for them in a "long tail". If your js is broken, well tough luck.
Are they just talking about how to implicitly handle commonly malformed HTML, as all browsers do?