Internet Explorer 7 and Internet Explorer 8 beta 2 do not support the javascript reserved keyword const used to define constants.

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Firefox 2, Opera 9, Safari 2, Safari 3.0.2, NS 6.1, NS 6.2, NS 7.0, NS 7.2 and many browsers support the javascript reserved keyword const to declare constants in javascript application.

reserved keyword const: "You can create a read-only, named constant with the const keyword. The syntax of a constant identifier is the same as for a variable identifier: it must start with a letter or underscore and can contain alphabetic, numeric, or underscore characters."

Update November 21st 2008:
August 26th 2008 ECMAScript 3.1 proposal suggests object.freeze instead to "sets the writable attribute to false for every own data property of the object.". const as a reserved word is deleted from ECMAScript 3.1 proposals and drafts.

Valid HTML 4.01!