Sustainability strategy at companies has become a key business and management aspect for the development and success of an enterprise. The communication of strategies and actions relating to sustainability has become increasingly important for both companies and brands. This research studies the communication process that forms part of the sustainable strategy of fashion companies, ranging from the corporate website to e-commerce, and it proposes improvements for sustainability communication. Two new models are presented: the Operational Model for Evaluating Fashion Corporate Websites (OMEFCW) and the Operational Model for Evaluating Fashion E-Commerce (OMEFeC), based on the core dimensions of online sustainability communication (orientation, structure, ergonomics and content—OSEC), as established by Siano. In order to obtain an optimal view of the fashion industry, four corporate groups—two luxury fashion groups (Kering Group and Moët Hennessy Louis Vuitton (LVMH)) and two fast fashion groups (H&M Group and Intidex)—are compared. In addition, all of the e-commerce operations of the groups’ fashion brands are analyzed, a total of 32 brands. The results show that it is necessary to continue improving in terms of the communication of sustainability within the fashion industry, whilst demonstrating the great deficiency that exists regarding the communication of sustainability in the case of the brands’ e-commerce operations, which are precisely the web pages most visited by consumers.