has the open-source WebKit rendering engine which Google characterizes as "HTML5, CSS and JavaScript". As far as I can determine, all of the features of JavaScript as it is today are implemented in WebKit, without (of course) any regard to the threat that some of them pose to computer system security.

At the moment, I cannot recall which other browsers use WebKit, too, but if Firefox doesn't use it yet, I suspect that Firefox 4 will.

In my experience, there are far too many websites that do not work well (or completely) with anything but versions of Internet Explorer. Microsoft encourages "customization" for I.E. because they are still convinced that they must, even if they can't, seize control of the Internet with their web browser.