It used to be that 90% of the time, you accessed stuff only on your computer’s local network. Then in 1993 at the University of Illinois, Marc Andreessen developed Mosaic, the first internet browser, which allowed users to wander around the World Wide Web 90% of the time. He moved to Silicon Valley and founded Netscape. Mr. Andreessen is now a general partner at a top-decile venture-capital firm, better than 90% of its peers. Andreessen Horowitz recently raised $9 billion in new funds.