This is a follow-up from bug 11137.

With bug 11137, we are making <input type='email' multiple>'s value easier to use because tokens have no longer leading or trailing whitespaces. However, there is still an issue when such a control is ended by a comma which can easily happen if there is a UI (with list or autocompletion for example).

I think there are two solutions
 1. Add a step 4 in "Whenever the user changes the element's values, the user agent must run the following steps:" which would consist of removing the last character if it's a comma;
 2. Make such control valid if it's ending with a comma.

The first solution seems better because the submitted value will be more simple to use because splitting on comma would not return a useless empty value at the end. But, for what I understand, with the latest change made on the specifications, it's still possible to directly change the value in the DOM and in the UI while the user is typing (it might be weird but that is an option). With such a change, it wouldn't be an option anymore.