Browsers need to translate instructions using a current formal spec, whilst being forgiving to human error and allowing backward compatibility with past specs /

Browsers need to translate instructions using a current formal spec, whilst being forgiving to human error and allowing backward compatibility with past specs / browser quirks. They also need to evolve quickly to meet future potential spec requirements. I'm no expert in the development of browsers; but I'd hazard a guess that the problem isn't as straightforward as it seems… and performance will suffer as a result, especially since all this has to occur at runtime.