You can validate the syntax of your CSS document from the menu Tools->Validate and dump or Tools->Validate only.
The validator uses a subset of the libcroco library modified to get its output in cssed.
Libcroco is a parsing and layout engine used to parse and show CSS styled output. Since it works exactly as a user agent - parsing until fatal error and avoiding some invalid values - we can know how our style sheet will be read from a CSS2 compliant UA.
In the menu Tools->Validate and dump you can check this behavior. It will show in the output window what it is reading and when it stops parsing.
The menu Tools->Validate only will only show the validation results.
This validator is unfinished, and some strange behavior may occur.
Especially if you use empty declarations, the validator will fail even if it must be ignored as it is valid CSS.