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Working on a new CSS feature like Container Queries, one of the most important considerations is to ensure a “migration path” – a way for developers to start integrating the new code, without breaking their sites on legacy browsers.

A large, carved, wooden hand
supporting a tree that has nearly fallen over
photo by Neil Thomas on Unsplash.

That looks different depending on the feature, but can often include new tests for the @supports conditional rule. So how do these CSS feature queries work, what happens when we add new syntax to the @supports rule, and anyway: what is the opposite of an unknown?