In an unprecedented move the Croatian national broadcaster HRT has restricted their call for the 2013 Eurovision Song Contest entry in the style of traditional “klapa” music. Submissions are accepted by Croatian nationals 30 days after the announcement on the HRT website.
Klapa is a traditional male a cappella singing, whose roots are in littoral church singing, and it is very popular in Croatia.
Complete rules and application forms are available on the HRT website here (in Croatian).
Source: hrt.hr, Wikipedia