I’m not sure that there would be a good reason to do it to be honest. Each engine is pretty mature and has been built into its design philosophy for years - nearing decades. If you have a different idea for how a browser works you’re probably still better off starting from scratch than trying to hodge podge your vision from an existing engine that is already unwieldy and has plenty of vulnerabilities (while more and more will appear as time goes on).
I’m not sure that there would be a good reason to do it to be honest. Each engine is pretty mature and has been built into its design philosophy for years - nearing decades. If you have a different idea for how a browser works you’re probably still better off starting from scratch than trying to hodge podge your vision from an existing engine that is already unwieldy and has plenty of vulnerabilities (while more and more will appear as time goes on).