With servo, it is pretty much a dead product at this point. It was primarily a research project in Mozilla, and with some recent pruning that Mozilla has done that leaves Servo inactive. It does still live on in Firefox as Firefox slowly migrates various components to Rust.

At this point, it is nearly impossible to write a from scratch engine from what I have heard. The number of standards and idiosyncrasies are too immense for it to be economical. Sure you can pass this test or that, but getting it to work on all sites is going to be much harder.