[css-animations] behavior of name attribute is not defined

The current spec says:
This attribute is the name of the keyframes, used by the animation-name
property.

However, there is no detailed behavior defined for getter and setter. To be
more exact, there is one question to be answered: should the setter accept
arbitrary unescaped string?

Since according to the spec, animation-name accepts a <custom-ident>, which
has some keywords excluded, I don't think the setter should accept
arbitrary string -- it should only accept valid <custom-ident>. As
<custom-ident> is identifier where identifier is an escaped sequence, the
getter should also return an escaped string instead of the unescaped
version.

Regards,
Xidorn Quan