I never worked on a browser, though I did implement the HTML5 spec, twice, and I wrote a JS engine(compiler, runtime). I know that it was a lot easier because I could count on infromation and other advances that were available to me, and anyone else really, at the time, the kind of resources and information I probably wouildn't have had access to it("whats javascript?") 20+ years earlier.

Browsers have evolved - because standards did, and expections along with them, but in the past they too were quite complex(see also Netscape's play: browsers, email, usenet client etc, suit) and again, it was harder then that it is nowadays to build such systems.