The concept of task completion in IndieWeb also reminds me a little of the functionality GoodRead.com has for updating one's status on reading a book: Either I'm on page X of Y or I'm X% done with book. Eventually one completes the book. In their UI, they also present a completion bar with the percentage of the book that's read to give a visual indicator as well. I don't suppose there's any reason a webmention couldn't also include a percentage finished to add this piece of data.