This article is well worth a read (and features Campaign Monitor).
It is a solid read, and I agree except for
# Use CSS sparingly. In most cases it is better to use inline CSS-styling in HTML instead of referring to CSS-file in HTML. However, referring to external CSS-files is better than sending them with newsletter.
External css files are stripped out by several big email clients.
True, but they didn't really recommend external css files (meaning files linked in from a url).
They recommended inline css, but said external css files were better than attached css files.
That's how I interpreted it, at least.
I'm not sure how well supported external nor attached css files are by email clients, but I think we can agree that a newsletter should be "self contained" as far as markup and css goes.
Could be a poorly worded phrase certainly. For information on support for linked styles see our chart.