What is this special behavior with document.cookie? Setting document.cookie to a plain value (no key)

So far this behavior is in Chrome and Firefox (latest versions of each).

Let’s say we set some cookies with the following JavaScript:

document.cookie = "is_halloween=true";
document.cookie = "skeletons=spooky";

Now let’s break the rules and do this:

document.cookie = "BOO";

If we check the value of document.cookie in the console, it looks like this:

is_halloween=true; skeletons=spooky; BOO

What exactly is BOO? If we try to remove it with the following code, nothing happens:

document.cookie = "BOO=;expires=Thu, 01 Jan 1970 00:00:00 GMT;";

document.cookie is still equal to is_halloween=true; skeletons=spooky; BOO

If we set document.cookie to an empty string:

document.cookie = "";

document.cookie is still equal to is_halloween=true; skeletons=spooky; BOO

Questions:

1. Is this special behavior in document.cookie intended to be used as a free text field for custom cookie syntax? If so, is it documented anywhere?

2. How do I get rid of BOO? The normal method of setting the expires field isn’t working.

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Edit: BOO can be removed with document.cookie="Boo;expires=Thu, 01 Jan 1970 00:00:00 GMT"