.conditionText is a single string, but there are two parts to the container query: the container name, and the query against it. Both of these form the condition of the rule, but it might be useful to have them represented independently in the API.
Suggestion: .conditionText is the entirety of the preamble, add .containerName and .containerQuery to represent the container name and query independently in the CSSContainerRule interface and the ContainerQueryList interface.