First of all - sorry if I missed it in the spec!

The suggestion is for simple memcpy/memset instructions, so copying/clearing can be done at maximum speed for any given platform/browser. For example bounds-checking doesn't have to be done pr. read/write, but only once. (Could be an issue on some implementations I think. Maybe also the polyfill...)

The memcpy could be provided in two versions, one that checks for overlaps and one that doesn't.