I don't understand why you would want to implement HTTP caching in a language's runtime. Honestly, I don't even see where and how you could even do that.
You don't implement HTTP caching, except in very specific cases.
You cache generated data where you need it. This is 1) Easy to add later, 2) Doesn't sacrifice the flexibility of being able to recompose that data on-demand, and 3) Doesn't require a bunch of complex front-end infrastructure to support.
Yes, and I was talking about implementing caching of components used to compose the pages and API responses, not a full HTTP cache tier that has to be placed in front of your application.
You cache generated data where you need it. This is 1) Easy to add later, 2) Doesn't sacrifice the flexibility of being able to recompose that data on-demand, and 3) Doesn't require a bunch of complex front-end infrastructure to support.