[css-animations] The `.style` attribute on CSSKeyframeRule should probably look more like other `.style` attributes that return `CSSStyleDeclaration`s.

[bzbarsky]

Specifically, https://drafts.csswg.org/css-animations/#interface-csskeyframerule says:

readonly attribute CSSStyleDeclaration style;

while other CSSOM bits spec it like so:

[SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;

The latter form is what appears in https://drafts.csswg.org/cssom/#the-csspagerule-interface and https://drafts.csswg.org/cssom/#the-cssstylerule-interface and https://drafts.csswg.org/cssom/#the-elementcssinlinestyle-interface (implemented by HTMLElement and SVGElement).