tailwind-merge applies some heuristics to detect the type of a class even if that particular class does not exist in the default Tailwind config. E.g. the class text-1000xl does not exist in Tailwind CSS by default but is treated like a font-size class in tailwind-merge because it starts with text- followed by an optional number and a T-shirt size, like all the other font-size classes.
This behavior has the advantage that you're less likely to need to configure tailwind-merge if you're only changing or extending some scales in your Tailwind config. But it also means that tailwind-merge treats classes that look like Tailwind classes as Tailwind classes although they might not be defined in your Tailwind config.
tailwind-merge detects the type of class by parsing the class name. When using a class like bg-[30%_30%], tailwind-merge can't know whether the class is a background-position or background-size class.
Therefore it is necessary to always prepend arbitrary values of background-position classes with the label position: as in bg-[position:30%_30%]. The same applies to background-size classes which need to be prepended with length:, size: or percentage:.
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