Is it a reference? If it is I did not get it, but I also did not miss it at all because I felt that it was quite clearly established as a shorthand for "improvements in the state of the art of writing software in general and specifically browsers, accumulated over the past two decades, which I don't want to describe here in detail, trust me that they exist or not, your choice" on first use. That shorthand was a considerable reading speedup for me, reference or not (if it is a reference, depending on what it references it might make reading slower for those who do get it - maybe I was just lucky?).