Fully sandboxing an unsafe renderer might have unacceptable performance. E.g. you'd have to reset the internal state after every call, otherwise invalid text on

Fully sandboxing an unsafe renderer might have unacceptable performance. E.g. you'd have to reset the internal state after every call, otherwise invalid text on a phishing website might be able to subvert the renderer to make it render text in the URL bar to read something different.