https://golang.org/src/net/http/server.go#L139
I would've expected the signature to be Write([]byte) error, not Write([]byte) (int, error). I also can't find any good explanation from looking through usages, and the documentation comment doesn't explain the return values.
What does the returned int mean?
The ResponseWriter.Write() method is to implement the general io.Writer interface, so a value of http.ResponseWriter can be used / passed to any utility function that accepts / works on an io.Writer.
io.Writer has exactly one Write() method and it details exactly the "contract" of Write, what it should return and how it should work:
type Writer interface {
Write(p []byte) (n int, err error)
}
Write writes len(p) bytes from p to the underlying data stream. It returns the number of bytes written from p (0 <= n <= len(p)) and any error encountered that caused the write to stop early. Write must return a non-nil error if it returns n < len(p).