I was reading this article the other day and found this...

Some email clients may behave in a very strange way when you remove the tabs or spaces in front of the CSS selector in the HEAD of your HTML.

Mailchimp have a similar statement in an article

We've noticed that some email servers (not MailChimp's, but your recipients') like to strip out any lines that begin with periods.

Both of these are pretty vague as to which email clients (or servers), have these issues. I've run a couple of quick tests and everything seemed to work ok.

Can anybody reproduce these errors? Or could it be an old issue that doesn't apply anymore?