This page aims to document the Unicode coverage of all the fonts (including non-free ones) in Debian. It was created using fontforge, font-manager and gucharmap. You should also read the DebianInstaller/GUIFonts page to find out about the fonts used by the installer.
Please add any fonts which might fill in some of these blocks to the Fonts/Missing page.
Iceweasel seems to render all Wikipedia languages:
Arabic mathematical alphabetic symbols: U+1EEF0 and U+1EEF1 missing
Armenian: U+058D and U+058E only available in Unifont
Elbasan: U+10500 and U+10502 to U+10527 missing
Georgian: U+10C7, U+10CD and U+10FD to U+10FF only available in Unifont
Georgian supplement: U+2D27 and U+2D2D only available in Unifont
Limbu: U+191D to U+191E only available in Unifont
Myanmar extended-A: U+AA7C to U+AA7F only available in Unifont
Old Hungarian: U+10C81, U+10C89, U+10C8B, U+10C90 to U+10C92, U+10C94 to U+10C98, U+10C9D to U+10C9F, U+10CA1, U+10CA8, U+10CA9, U+10CAC, U+10CC0 to U+10CF2 and U+10CFA to U+10CFF missing
Old Permic: U+10351 to U+10354, U+1035A to U+1035E, U+10363 to U+10365, U+10368 to U+1036C, U+1036E and U+10370 to U+1037A missing
Sinhala: U+0DE6 to U+0DEF only available in Unifont
Telugu: U+0C00 and U+0C34 only available in Unifont
Vedic extensions U+1CF8 to U+1CF9 only available in Unifont
Completely empty blocks
Some blocks have bitmap glyphs available in Unifont and Unifont Upper, noted with an asterisk (*) below.
Bassa Vah (U+16AD0...)
Caucasian Albanian (U+10530...)
Cherokee supplement (U+AB70...)
Mende Kikakui (U+1E800...)
Myanmar extended-B (U+A9E0...) *
Old North Arabian (U+10A80...) *
Pahawh Hmong (U+16B00...) *
Pau Cin Hau (U+11AC0...)
Psalter Pahlavi (U+10B80...) *
Sinhala archaic numbers (U+111E0...)
Sora Sompeng (U+110D0...)
Sutton signwriting (U+1D800...)
Warang Citi (U+118A0...)
Some conscripts in the ConScript Unicode Registry (such as Klingon, Tengwar and Visible speech) have bitmap glyphs available in Unifont CSUR and Unifont Upper CSUR.