The difference in the amount of attention invested in spec support in Chrome and Safari can be explained by the difference in business models between Google and

The difference in the amount of attention invested in spec support in Chrome and Safari can be explained by the difference in business models between Google and Apple. For Google, the browser is closely associated with their revenue streams. If you profit by showing ads across web properties, you’d naturally prefer to have more control over how those properties are viewed. The motivation can be benevolent in some aspects (to show more ads, we want people to want to browse the Web more—so we want to support fancier tech enabling fancier websites to be developed more easily) and less so in others (we want to control the viewing mechanism to ensure people keep seeing ads; we want to become the de-facto standard of browsing so that we don’t have to worry some newcomer eating at our revenue stream; the more complex the spec, the bigger our moat). To Apple, the browser means relatively little on desktop; on the more sandboxed mobile OS it matters more due to being responsible for a lot of the attack surface.