Outlook 2007 ignoring CSS within an HTML email due to 2 semicolons (;;)

Getting HTML emails looking good in Outlook 2007, is a relentless task at the best of times, but this one really caught me out...

The system I was debugging is used for creating and sending Email Marketing campaigns.

When creating a campaign, the system allows you to pick a template, - which selects a CSS style sheet which gets embedded into a style tag at the top of the email.

With one particular template, only around 10% of the styles appeared to be getting applied to the email - most of the email was just using the default styles.

When investigating, I noticed that all the CSS up to a particular point in the file (about 10% of the way through) was being applied, and the rest was being ignored.

This line caused the problem:

All CSS after the double semicolon was ignored.

The strange thing is, the CSS file validated successfully using the W3C validation service...

Interesting feature...