In strict mode code:
Historically many ECMAScript implementations have included extensions of this sort. In some cases (for example FunctionDeclarations in Statment contexts) the details of the extensions differ enough between implementations that there is little or no useful intersection semantics among the various implementations of a seemingly common extension.
ES5 strict mode, is a completely new mode of the language for which no usage currently exists. This means that it can be a clean foundation for the future evolution of the lanaguage. For this reason, TC39’s plan of record is limit most future feature additions to operating in strict mode. However, this plan may not work if ES5 implementations pollute strict mode with inconsistent and incompatable implementations of extensions that are likely to conflict with future standardization efforts.
TC39 already has tentative agreement for including const, let, and statement level function declarations in the next edition of ECMAScript. For this reason, ES5 implementation should avoid including these features in strict mode until complete specifications of these features have been written and approved by TC39.