Why are some object-literal properties quoted and others not?

Question

I see this all the time: object literals declared such that some keys are surrounded with quotes and others are not. An example from jQuery 1.4.2:

jQuery.props = {
    "for": "htmlFor",
    "class": "className",
    readonly: "readOnly",
    maxlength: "maxLength",
    cellspacing: "cellSpacing",
    rowspan: "rowSpan",
    colspan: "colSpan",
    tabindex: "tabIndex",
    usemap: "useMap",
    frameborder: "frameBorder"
};

What is the significance of wrapping the first two property keys (for and class) with quotes, while leaving the others quote-less? Are there any differences at all?

I've been poking around the ECMAScript 5 specification; all I've been able to find is [Note 6 of Section 15.12.3, emphasis mine]:

NOTE 6 An object is rendered as an opening left brace followed by zero or more properties, separated with commas, closed with a right brace. A property is a quoted String representing the key or property name, a colon, and then the stringified property value. An array is rendered as an opening left bracket followed by zero or more values, separated with commas, closed with a right bracket.

However, this refers only to the stringification of JSON.

Answer 1

Those are Javascript reserved words, and (though not really necessary) the syntax of the language requires that they be quoted.

Strictly speaking, pure "JSON" notation requires that all of the "key" strings be quoted. Javascript itself however is OK with keys that are valid identifiers (but not reserved words) being unquoted.