2.1.2 [ECMA-262/5] Section 10.1.1, Strict Mode Code
The specification states:
Strict Mode Code An ECMAScript Program syntactic unit may be processed using either unrestricted or strict mode syntax and semantics.
IE9 Mode, IE10 Mode, and IE11 Mode (All Versions)
No code is interpreted as strict mode code. For more information about strict mode, see Annex C of [ECMA-262/5].