Browser implementation teams actually do sometimes refuse to implement a feature of the applicable standards. It's pretty rare, though.
The Webkit team's refusal to implement customized built-in elements sticks in the mind, not least because their reasons (and inability to describe an alternative) always sound more like a vendor pissing contest than any kind of engineering discussion.
The Webkit team's refusal to implement customized built-in elements sticks in the mind, not least because their reasons (and inability to describe an alternative) always sound more like a vendor pissing contest than any kind of engineering discussion.