in reply to
Re: How to extract an email address from a mailto URL?
in thread How to extract an email address from a mailto URL?
Also, older HTML allowed you to not quote the information after the equal sign in some circumstances.
Newer HTML too; it is only XHTML that makes quoting attribute values mandatory in all circumstances.
Since email addresses will include an @ character, the attributes are mandatory in this case.
That said, it isn't as if nobody ever breaks the rules, so it is generally a good idea to write code that can cope (unless you know that the incoming data won't have that problem).