I'm reading the Azure Patterns and Practices multi-tenant guidance and in there it says
Cascading style sheets Behaviors are one feature that the Surveys site will not support.
What CSS features should I support in a multi tenant website?
There are two issues.
In principle, you could write a CSS sanitizer to defend against these attacks and still allow tenants to specify their own CSS. However, that is quite tricky to do right and it would be easy to introduce a mistake, so it's not something to undertake lightly.