Created attachment 485028 [details] [diff] [review] Patch v1 <input type='email' multiple>'s value has to be a set of comma-separated tokens  and all tokens have to be valid email addresses. This make "email@example.com, " not valid because the empty string isn't a valid email address. The problem is, as long as "," isn't typed, the user is still editing the current email address. So, if we want to have a UI to select addresses, the UI will have to add "firstname.lastname@example.org, " so the user can select multiple addresses easily. However, this will make the field invalid because of the ending comma. Not that with this patch "email@example.com, " is valid but not " , " because even if it's ending with a comma, the first email address is not valid (the empty string).  http://www.whatwg.org/specs/web-apps/current-work/multipage/common-microsyntaxes.html#set-of-comma-separated-tokens
I will push that if bug 601209 can be fixed for Gecko 2.0. No need to push this patch otherwise.
This will be done after Gecko 2.0 given that bug 601209 will not be done for Gecko 2.0.