Clarification of issue in User Timing spec

In the *Exceptions* section of the User Timing spec for the mark method, it
states:

"Throws a SYNTAX_ERR exception if the markName argument is the same name as
an attribute in the PerformanceTiming interface."

In Gecko we implemented bug 1047878 [1] out-of-spec and added `toJSON` to
the PerformanceTiming interface to get a JSON version of the timing
attributes. Following this reasoning, then `toJSON` being a
PerformanceTiming attribute may need to throw a SYNTAX_ERR when used as a
performance marker name:

performance.mark('toJSON');

So we need some clarification of the spec to determine whether all
attributes of the PerformanceTiming interface should throw if used as
marker names, or only those specifically defined by the spec.

Thanks!

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=760851

Eli Perelman
Mozilla