WebKit, Chromium and Gecko all have a limit on the size of the session history entry list, WebKit uses 100 and Chromium and Gecko use 50, and adding more entries will start dropping entries from the front of the list. I think the spec should at least mention that there might be a limit.

We're still trying to figure out how this affects the various algorithms, but for example step 6 in https://html.spec.whatwg.org/#url-and-history-update-steps simply sets length to index + 1, which is not what browsers currently do when that goes over the limit.

@jakearchibald @smaug----