Even though several people seconded the complexity of a browser, I must add one more take and bring up one of my all time favorite blog posts, back from 2000 (I am old), when browsers were already complex, Joel Spolsky's Joel On Software episode "Things You Should Never Do, Part I"
https://www.joelonsoftware.com/2000/04/06/things-you-should-...
His first example was Netscape browser v6.0, and why there wasn't a v5.0 after v4.0, why it took three years: "They did it by making the single worst strategic mistake that any software company can make:
They decided to rewrite the code from scratch."
I think this blog post is very relevant here.