Porsche Boxster Owner's Manual page 53

Safety belts
If it becomes necessary to clean the belts, you can
use any mild washing agent. Allow the belts to dry
prior to retracting, but avoid direct sunlight.
Only use suitable cleaners. If unsuitable
cleaners are used or any attempt is made
to dye or bleach the belts, the webbing may be
weakened and thus constitute a safety risk.
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Controls, Instruments