Is it still a requirement that an (online) encyclopedia has to be balanced?
By balance, critics of encyclopedias think of the content. If the publisher advertises a reference work of general interest he is supposed to deliver a balanced compendium on different subjects.
Many famous encyclopedias have been less balanced or universal as one might expect. Pliny’s Naturalis historia dealt with ‘cultural’ subjects as it did with ‘natural’ subjects, but obviously the bulk of the content was about the earth and its living beings, about crafts and agriculture. The visual arts found a place next to metallurgy and mineralogy.
In the 17th and 18th centuries, and even beyond, encyclopedias belonged usually to one of two groups:
- ‘Dictionaries of the arts and sciences’ were occupied mainly with nature and crafts. Note that science then included also theology. Examples: Lexicon technicum (Harris), Cyclopaedia (Chambers), Deutsche Taschen Enzyklopädie (Hasse).
- ‘Dictionaires historiques’ and ‘Konversationslexika’ had their focus on history, geography and biography. Examples: Grand dictionnaire historique (Moréri), Ersch-Gruber.
According to Ulrike Spree the general lexika in Germany of the early 19th century wanted to serve the development of a learned conversation, as a base for the upcoming public opinion. These lexika were interested more in history and politics than science (in the modern sence), and had to be up-to-date. In Britain, it was the other way round. Encyclopaedia Britannica (EB) started in 1768 as a dictionary of the arts and sciences, only later the publishers added geography, history and so on. ‘Not the significance at a given time, but the importance of a subject by itself, had to be decisive for the inclusion of an article into the lexicon.’ Later in the supplements after 1824 there was much more attention to politics. 
It remained difficult for encyclopedias to keep an overview and maintain balance. Harvey Einbinder criticises how in the EB of 1958 the presidency of Calvin Coolidge is given more space than Franklin D. Roosevelt’s with his dramatic life, ‘a balance that reflects the EB’s haphazard editorial standards’. 
Just recently, a German professor of law complained in his vademecum for students that there is no balance in Wikipedia. ‘Homer Simpson’ (the cartoon character) has more lines than ‘Homer’ (the ancient author), and ‘Robbie Williams’ more than ‘Béla Bartók':
As long as inclusionists want to collect as much information as possible also about obscure subjects (there is enough space for everything), while exclusionists want to keep up quality and relevance (not everything has to be mentioned in a lexicon), a unified policy will hardly be found. (Roland Schimmel) 
There exists also another kind of lack of balance: Jeff Loveland finds it striking how ambitious encyclopedias were at the beginning, and how much impatient and sloppy at the end. Take Encyclopaedia Britannica, first edition. A-B delivers as much material as M-Z does. In the Universal history by Dennis de Coetlogon one-third of the text appeared under A-C. 
Because of a major difference, the readers of Wikipedia don’t have to bother much. When you bought a printed encyclopedia you had the right to ask for a good ‘balance’. But nobody buys Wikipedia, and the reader is happy when he finds enough about the subjects he is interested in. It doesn’t matter to him that other subjects are covered much better.
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 Jeff Loveland: An Alternative encyclopedia? Dennis de Coetlogon’s Universal history of arts and sciences (1745). Voltaire Foundation, Oxford 2010, pp. 7/8.
 Ulrike Spree: Das Streben nach Wissen. Eine vergleichende Gattungsgeschichte der populären Enzyklopädie in Deutschland und Großbritannien im 19. Jahrhundert, Niemeyer 2000, pp. 36/37.
 Harvey Einbinder: The Myth of the Britannica. MacGibbon & Kee, London 1964 (reprint 1972), p. 148.
 Jeff Loveland: An Alternative encyclopedia? Dennis de Coetlogon’s Universal history of arts and sciences (1745). Voltaire Foundation, Oxford 2010, pp. 88/89.