This means that, for example, variables will refer to distinctly different values if their names differ in the case of any part of their names. Hence aaa is not the same variable as Aaa.
MSIE browsers prior to version 4 were less particular about case sensitivity. Since the ECMA standard requires strict case sensitive behavior, this is now the norm.
Some early versions of the WebTV set top box prior to the Summer 2000 release also lacked case sensitive behavior regarding built-in method and property names.
In MSIE version 3, all client-side object and property names were case-insensitive. Beware of any old scripts, which may have worked on MSIE version 3, but don't work on later browsers.
ECMA 262 edition 2 - section - 7.5
ECMA 262 edition 3 - section - 7.6