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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.

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?