if `declaration.property` already exists for `selector`
if old `declaration.value` is `!important`
if new `declaration.value` is !important`
override existing `declaration.value` for `selector`
else
keep existing `declaration.value` for `selector`
else
override existing `declaration.value` for `selector`
else
add `declaration.property` with `declaration.value` on `selector`
Figure out how to handle
!important
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