You can't simply hash the whole page, because for most sites the hash will change constantly due to any varying content (e.g. rotating ads, new comments, random messages, A/B testing...). I wrote up a proposal for using a CSS rule to select only the "relevant" portion of a page to hash.[1]

[1] https://bentrask.com/?q=hash://sha256/2c9e53858b5312564a2b8f...