P is a California corporation whose principal place of business is the Northern District of California, and whose three employees reside in said district. P 'is engaged in the business of research, design, development and manufacture of digital gas analyzer and depth gauge products . . . for the commercial diving industry.' P maintains a website 'on which it describes the external specifications of its products,' but through which it does not 'sell or offer [them] for sale.' D, a corporation that sells commercial diving helmets and surface gas controllers and analyzers sent P a cease and desist letter, by which D accused P of infringing its trademarks by 'using images and representations of Kirby Morgan's helmets on its website, Facebook page, t-shirts, Twitter and on the panels of P's gas analyzer equipment' and demanded that P cease its use of the infringing images. P sought a declaration of non-infringement of D's federally registered trademarks. D filed an action in the Central District, alleging, inter alia, federal trademark infringement claims against P.