I suspect that the primary initial target with "use stricter" mode will be backend services in io.js or node.js first, as these are more likely to benefit from

I suspect that the primary initial target with "use stricter" mode will be backend services in io.js or node.js first, as these are more likely to benefit from the optimizations than frontend code will. For now the list of web browser applications that really require that much speed and optimization is still pretty small. This new mode will make no difference at all for the client side code in your average Facebook, or Twitter. But having a faster more efficient static typing system will make servers much faster and allow more reqs/sec per server and therefore fewer servers overall, for huge savings.