2.1.5 [ECMA-262/5] Section 14.1, Directive Prologues and the Use Strict Directive
The specification states:
A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this occurs. NOTE: The ExpressionStatement productions of a Directive Prologue are evaluated normally during evaluation of the containing SourceElements production. Implementations may define implementation specific meanings for ExpressionStatement productions which are not a Use Strict Directive and which occur in a Directive Prologue. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a Directive Prologue an ExpressionStatement that is not a Use Strict Directive or which does not have a meaning defined by the implementation.
IE9 Mode, IE10 Mode, and IE11 Mode (All Versions)
Special meanings are not defined for ExpressionStatement productions that are not a Use Strict Directive and that occur in a Directive Prologue. In addition, a warning is not issued if a Directive Prologue contains multiple Use Strict Directives.