In the case of JavaScript, at the beggining it didn't just "came" with all browsers since it was first implemented by Netscape. Why MS included it in IE is far from my knowledge/interest into the history of JavaScript/MSIE. I guess that once your language is in 90% (Netscape + MSIE) of all platforms, then yes, all (Chrome, Safari, etc) had to follow along.
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In the case of JavaScript, at the beggining it didn't just "came" with all browsers since it was first implemented by Netscape. Why MS included it in IE is far from my knowledge/interest into the history of JavaScript/MSIE. I guess that once your language is in 90% (Netscape + MSIE) of all platforms, then yes, all (Chrome, Safari, etc) had to follow along.