CSS has the power to completely change the design of the page in which it is included, so, simple CSS rules in an email opened in Hotmail could easily drastically change Hotmail's interface. This could allow not well formed CSS rules in an email to disfigure the interface, or, worst, it could be used by ill-intentioned persons to mislead the user (spamming, phishing). So, to prevent those situations, web based clients "read" the email before it's showed and removes any CSS rule it considers a danger. The problem is that unwanted properties and CSS control rules differ from one client to the other.
A couple of the most important issues are:
- Gmail only keeps the HTML between the
