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CSS 2.1 was chosen for the Validation with W3C simply because I mistakenly thought that was the version used by EPUB 2. I mainly wanted to downgrade from CSS 3 in order to highlight some more obvious errors.
I just tried changing to CSS 2 on the W3C site but get some unexpected and undecipherable error message. So for now at least in the beta it will stay CSS 2.1. But if anyone who knows the spec and the site well can confirm it really should be CSS 2 let me know. It is only meant to be a helpful guide - and as everyone is aware, you still have to check on the actual readers to see what really happens.
In addition to FlightCrew picking up on "content" as the OP noted, I've noticed it also correctly picks up on 'background-image' not being valid. I've added an extra check in the Delete Unused Images / Files tool to check the CSS for references to images in that attribute (and 'background') - even though its not valid EPUB 2 its better to not delete it than to delete it.
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