Specifically, we want to be able to type http://site?ModPagespeedFilters=+debug 
to give you HTML with comment annotations indicating why the system made 
various decisions.

As a Proof Of Concept that was already done for combine_css.  So we already 
have a filter called "debug", a query-param mechanism to incrementally add a 
new filter by name, and an API to insert a comment near the current event while 
calling filters.

We just need to put more of this information into more filters.

One complexity with the CSS parser is that it is currently calling LOG(X) 
directly rather than going through something like our MessageHandler API where 
we can capture the results.  We'd need to refactor that in a way that would 
continue to work for the non-instaweb google3 users of the CSS parser.

More can always be done, but this is much improved as of 1.9.