In response to w3ctag/design-reviews#390

Also, I find inPlace confusing. It doesn't explain what inPlace does, and repeats the description of keepExistingData's behavior. Our understanding so far is that inPlace = true means it will write directly to the destination file without creating a temporary copy first, but it's not clear. So potentially faster, but I though the copy was there because of security and integrity - I understood you never wanted to write incomplete data.