It's an in-joke taking aim at Blink's stated 2014 goal of optimizing for mobile performance at the expense of supporting things like CSS Regions. Shadow DOM is also the subject of some controversy, as Blink and Mozilla seem intent on pressing ahead with implementing it without full buy-in from all vendors or even the WG.

Shadow DOM is a mechanism by which a browser can inject a subtree into the DOM, but it won't be visible to JS.

This was often used for things like custom elements (inputs and such). I presume it wasn't very popular to use the shadow dom in webkit.

Far as I recall, there's also talk about using the shadow dom for injecting foreign fragments into the DOM in a secure manner (web components). Although there's probably ways to achieve this without the full shadow dom.

> Can someone explain this?

They're finishing up the removal of unmaintained and essentially dead code which started after the Blink fork.

> Isn't Shadow DOM necessary?

To what?

> What is this 60fps on mobile intent and how it relates to Shadow DOM?

Shadow DOM requires hooks throughout the engine. If you remove the hooks (because nobody uses them because shadow dom is not supported) the engine is lighter and executes faster. Hence 60fps goal.