considered the work to be a waste of precious natural resources. Khan's work promoted structures integrated with architecture and the least use of material resulting in the smallest impact on the environment. The next era of skyscrapers will focus on the environment including performance of structures, types of material, construction practices, absolute minimal use of materials/natural resources, embodied energy within the structures, and more importantly, a holistically integrated building systems approach. Modern building practices regarding supertall structures have led to the study of "vanity height". Vanity height, according to the CTBUH, is the distance between the highest floor and its architectural top (excluding antennae, flagpole or other functional extensions). Vanity height first appeared in New York City skyscrapers as early as the 1920s and 1930s but supertall buildings have relied on such uninhabitable extensions for on average 30% of their height, raising potential definitional and sustainability issues. The current era of skyscrapers focuses on sustainability, its built and natural environments, including the performance of structures, types of materials, construction practices, absolute minimal use of materials and natural resources, energy within the structure, and a holistically integrated building systems approach. LEED is a current green building standard. Architecturally, with the movements of Postmodernism, New Urbanism and New Classical Architecture, that established since the 1980s, a more classical approach came back to global skyscraper design, that remains popular today. Examples are the Wells Fargo Center, NBC Tower, Parkview Square, 30 Park Place, the Messeturm, the iconic Petronas Towers and Jin Mao Tower. Other contemporary styles and movements in skyscraper design include organic, sustainable, neo-futurist, structuralist, high-tech, deconstructivist, blob, digital, streamline, novelty, critical regionalist, vernacular, Neo Art Deco and neohistorist, also known as revivalist. 3 September is the global commemorative day for skyscrapers, called "Skyscraper Day". New York City developers competed among themselves, with successively taller buildings claiming the title of "world's tallest" in the 1920s and early 1930s, culminating with the completion of the Chrysler Building in 1930 and the Empire State Building in 1931, the world's tallest building for forty years. The first completed tall World Trade Center tower became the world's tallest building in 1972. However, it was overtaken by the Sears Tower (now Willis Tower) in Chicago within two years. The tall Sears Tower stood as the world's tallest building for 24 years, from 1974 until 1998, until it was edged out by Petronas Twin Towers in Kuala Lumpur,