With the help of the ETag and the If-Match headers, you can detect mid-air edit collisions.

For example, when editing MDN, the current wiki content is hashed and put into an Etag in the response:

When saving changes to a wiki page (posting data), the POST request will contain the If-Match header containing the ETag values to check freshness against.

If the hashes don't match, it means that the document has been edited in-between and a 412 Precondition Failed error is thrown.