What makes implementation so difficult is that browsers cannot just “work”, they need to be correct is what they do. And support all websites.
The standards of HTML, CSS and JS have developed over a long time, not only is the amount of stuff massive, over time sometimes strange features where implemented, that were then used by website developers, and now these all need to be handled correctly by all new browsers.
Emulating and reimplementing existing stuff is often more difficult, especially if you cannot leave out any feature, no matter how obscure, because that might break someone’s website.
What makes implementation so difficult is that browsers cannot just “work”, they need to be correct is what they do. And support all websites.
The standards of HTML, CSS and JS have developed over a long time, not only is the amount of stuff massive, over time sometimes strange features where implemented, that were then used by website developers, and now these all need to be handled correctly by all new browsers.
Emulating and reimplementing existing stuff is often more difficult, especially if you cannot leave out any feature, no matter how obscure, because that might break someone’s website.