Art Deco emerged in Paris in the 1920s, crystallized at the 1925 Exposition Internationale des Arts Decoratifs. It fused the geometric abstraction of Cubism with the luxury materials of traditional French decorative arts — and then it went everywhere. The Chrysler Building. Miami Beach hotels. Hollywood movie palaces. Jewelry, furniture, fashion, ocean liners.
What made it spread was its versatility within constraint. The grammar is strict — symmetry, geometry, stepped forms, radiating lines — but it can be applied to anything from a skyscraper to a cocktail shaker. It's ornament that follows rules, which is why it translates so cleanly to CSS. The sunburst is a gradient. The stepped form is nested divs. The gold border is a border.
On the web, Art Deco appears whenever a brand needs to signal sophistication without stuffiness. It's the aesthetic of the boutique hotel landing page, the awards show microsite, the luxury product reveal. It never really went away — it just waits for occasions worthy of its precision.