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Library of Congress
Loc: The Moldenhauer Archives
The Moldenhauer Archives at the Library of Congress contain approximately 3,500 items documenting the history of Western music from the medieval period through the modern era.
OpenStax
Open Stax: Catherine Schmidt Jones: The Music of the Romantic Era
Here is a nice introduction to the Romantic Era. It describes the various elements and the composers of this time.
Other
Music History Online: Music of the 19th Century
Learn about the various styles of music from the 19th century. From romanticism to nationalism, this website goes into detail about the history of 19th century music.
Classical Net
Classical Net: Musical Periods
This site contains the most significant artistic output of the most important composers within each historical period.