IIRC Netscape didn't either around the IE4/5 days. It also had significant performance problems with large sets of nested tables which IE6 (and perhaps IE5) did not.

For example back at University on 133MHz Pentium-class CPUs a large slashdot page with all threads open could take minutes to render where IE took merely a noticeable number of seconds - a co-student use to make a big thing of this as a reason to dump other browsers and use IE. There was actually a use case for this away from the University's fixed line connection: on dial up you could, RAM+swap and browser-not-crashing permitting, open a couple of windows like that then disconnect the modem and read at your leisure without racking up a per-minute phone bill.

Or maybe NS was capable of progressive display and this was just hidden in extreme cases by the performance issue, that was a time ago and memory is hazy…