India, April 17 -- On first glance, a collaboration between Stella McCartney and H&M might feel unexpected but it isn't new. If anything, it's one of the earliest examples of how high fashion and mass retail learned to meet in the middle, with H&M acting as a platform that could take a designer's point of view and scale it without flattening it entirely.

That exchange began in 2005, when McCartney partnered with H&M for what was only its second-ever designer collaboration, following Karl Lagerfeld's debut. At the time, designer collaborations were still an experiment. McCartney agreed on one condition: sustainability had to be part of the foundation, not an add-on. That meant working with better materials, introducing early conversations...