User Timing Mark Name for "Critical Content Loaded"?

There are some recommended mark names
<http://www.w3.org/TR/user-timing/#introduction-1> in the user timing spec
but none of them seem to be a good fit for what I am trying to track.  I'd
like to see a well-defined name that sites can use to mark when they
believe their critical content has loaded.  i.e.:

- "Time to first tweet" for twitter
- "Time to first pin" for pinterest
- Time when the hero image loads for an e-commerce or news site

"mark_above_the_fold" is the closest but is semantically quite different.
Any chance we can update the spec with one more recommended name?
Something like "critical_content_loaded" (not sure why the others are
prefixed with mark_ since they are all marks by definition)?

Understanding that it's just convention and nothing is guaranteed, a
standard convention for "this is the time I care about most" would be
helpful for a bunch of cases:

- RUM providers could automatically collect it and report on it as a
special case
- Synthetic testing (like WebPageTest) could extract it and report on it as
a top-level metric
- Browsers could collect it as part of their field metrics and optimize for
it

The last case in particular is one that I'd like to implement in Chrome.
We track all marks right now as a histogram but the data isn't terribly
useful because we don't really know what site developers are tracking and
onload is not terribly useful to optimize for. Having developers (even a
small set of high-profile sites) instrument a well-known "optimize for
this" metric would be significantly more useful.

We could advertise a name that we'd be tracking but I think it would be a
lot more impactful if we could have the standard name added to the spec and
just point to that.

Thanks,

-Pat