Initially Netscape Open Sourced Mariner that was on track to becoming the engine of Netscape 5 and was an evolution of the engine of Netscape 4. Netscape 4 lack

Initially Netscape Open Sourced Mariner that was on track to becoming the engine of Netscape 5 and was an evolution of the engine of Netscape 4. Netscape 4 lacked proper DOM support and proper CSS support. (Project to add DOM support was called Perignon, IIRC.) Netscape had purchased another company called Digital Style. Their layout tech became Gecko but was called Raptor and NGLayout before being renamed to Gecko. Mariner / Netscape 5 was canceled and Mozilla migrated to Gecko, which was used in Netscape 6. SpiderMonkey survived the transition from Mariner to Gecko and is used in Servo, too.