Kobold letters (via) Konstantin Weddige explains a sophisticated HTML email phishing vector he calls Kobold emails.
When you forward a message, most HTML email clients will indent the forward by nesting it inside another element.
This means CSS rules within the email can be used to cause an element that was invisible in the original email to become visible when it is forwarded—allowing tricks like a forwarded innocuous email from your boss adding instructions for wiring money from the company bank account.
Gmail strips style blocks before forwarding—which it turns out isn’t protection against this, because you can put a style block in the original email to hide the attack text which will then be stripped for you when the email is forwarded.