Is there any theoretical reason why client-side JavaScript could not be spec’d to allow for direct access to POST or PUT contents without need for a server language to write it into client-side JavaScript first?

It would seem that this, besides avoiding an unnecessary dependency on server-side code where not needed, would allow for arbitrary length command-line type operations with side effects (such as changing localStorage) and could also open up what could be done with HTML served over the file:// protocol (assuming the POST to the file:// originated from a trusted context).