Simplify the following text for a Grade 11 native English-speaking person.
Original: Art Nouveau monuments are now recognized by UNESCO on their World Heritage Sites list as significant contributions to cultural heritage . The historic center of Riga , Latvia , with `` the finest collection of art nouveau buildings in Europe '' , was inscribed on the list in 1997 in part because of the `` quality and the quantity of its Art Nouveau\/Jugendstil architecture '' .
Simplified: As the Art Nouveau matured emphasis on vertical lines became more popular , known as `` Vertical '' Art Nouveau this style was most popular shortly before the World War I. The center of Riga is now designated as UNESCO World Heritage site in part for its Art Nouveau architecture .