Checks if a value is an instance of a class or a class name. Useful when you want to check if a value is an instance of a class that may not be defined in the current scope. For example, if you want to check if a value is an OffscreenCanvas instance, you might not want to do the song and dance of using typeof OffscreenCanvas !== 'undefined' and then shimming OffscreenCanvas if the types aren't around.