A composition comprising a first host compound represented by Chemical Formula I and a second host compound represented by Chemical Formula II: wherein, in Chemical Formula I, X 1 is O, Se, S, SO, SO 2, PO, or CO, X 2 is CR a R b or SiR c R d, and R 1 to R 5 and R a to R d are each independently hydrogen, deuterium, a substituted or unsubstituted C1 to C30 alkyl group, a substituted or unsubstituted C3 to C30 cycloalkyl group, a substituted or unsubstituted C3 to C30 heterocycloalkyl group, a substituted or unsubstituted C6 to C30 aryl group, a substituted or unsubstituted C3 to C30 heteroaryl group, a substituted or unsubstituted amine group, a substituted or unsubstituted C6 to C30 arylamine group, a substituted or unsubstituted C3 to C30 heteroarylamine group, a substituted or unsubstituted C1 to C30 alkoxy group, a substituted or unsubstituted C2 to C30 alkoxycarbonyl group, a substituted or unsubstituted C2 to C30 alkoxycarbonylamino group, a substituted or unsubstituted C7 to C30 aryloxycarbonylamino group, a substituted or unsubstituted C1 to C30 sulfamoylamino group, a substituted or unsubstituted C2 to C30 alkenyl group, a substituted or unsubstituted C2 to C30 alkynyl group, a substituted or unsubstituted silyl group, a substituted or unsubstituted silyloxy group, a substituted or unsubstituted C1 to C30 acyl group, a substituted or unsubstituted C1 to C20 acyloxy group, a substituted or unsubstituted C1 to C20 acylamino group, a substituted or unsubstituted C1 to C30 sulfonyl group, a substituted or unsubstituted C1 to C30 alkylthiol group, a substituted or unsubstituted C1 to C30 heterocyclothiol group, a substituted or unsubstituted C6 to C30 arylthiol group, a substituted or unsubstituted C1 to C30 heteroarylthiol group, a substituted or unsubstituted C1 to C30 ureide group, a halogen, a halogen-containing group, a cyano group, a hydroxyl group, an amino group, a nitro group, a carboxyl group, a ferrocenyl group, or a combination thereof, wherein in Chemical Formula II, X 3 is O, S, CR u R v, SiR w R x, or NR y, HTU is a substituted or unsubstituted carbazolyl group or a substituted or unsubstituted triphenylenyl group, and R 11 to R 13 and R u to R y are each independently hydrogen, deuterium, a substituted or unsubstituted C1 to C20 alkyl group, a substituted or unsubstituted C6 to C30 aryl group, a substituted or unsubstituted C3 to C30 heteroaryl group, or a combination thereof. wherein, in Chemical Formula I, X 1 is O, Se, S, SO, SO 2, PO, or CO, X 2 is CR a R b or SiR c R d, and R 1 to R 5 and R a to R d are each independently hydrogen, deuterium, a substituted or unsubstituted C1 to C30 alkyl group, a substituted or unsubstituted C3 to C30 cycloalkyl group, a substituted or unsubstituted C3 to C30 heterocycloalkyl group, a substituted or unsubstituted C6 to C30 aryl group, a substituted or unsubstituted C3 to C30 heteroaryl group, a substituted or unsubstituted amine group, a substituted or unsubstituted C6 to C30 arylamine group, a substituted or unsubstituted C3 to C30 heteroarylamine group, a substituted or unsubstituted C1 to C30 alkoxy group, a substituted or unsubstituted C2 to C30 alkoxycarbonyl group, a substituted or unsubstituted C2 to C30 alkoxycarbonylamino group, a substituted or unsubstituted C7 to C30 aryloxycarbonylamino group, a substituted or unsubstituted C1 to C30 sulfamoylamino group, a substituted or unsubstituted C2 to C30 alkenyl group, a substituted or unsubstituted C2 to C30 alkynyl group, a substituted or unsubstituted silyl group, a substituted or unsubstituted silyloxy group, a substituted or unsubstituted C1 to C30 acyl group, a substituted or unsubstituted C1 to C20 acyloxy group, a substituted or unsubstituted C1 to C20 acylamino group, a substituted or unsubstituted C1 to C30 sulfonyl group, a substituted or unsubstituted C1 to C30 alkylthiol group, a substituted or unsubstituted C1 to C30 heterocyclothiol group, a substituted or unsubstituted C6 to C30 arylthiol group, a substituted or unsubstituted C1 to C30 heteroarylthiol group, a substituted or unsubstituted C1 to C30 ureide group, a halogen, a halogen-containing group, a cyano group, a hydroxyl group, an amino group, a nitro group, a carboxyl group, a ferrocenyl group, or a combination thereof, wherein in Chemical Formula II, X 3 is O, S, CR u R v, SiR w R x, or NR y, HTU is a substituted or unsubstituted carbazolyl group or a substituted or unsubstituted triphenylenyl group, and R 11 to R 13 and R u to R y are each independently hydrogen, deuterium, a substituted or unsubstituted C1 to C20 alkyl group, a substituted or unsubstituted C6 to C30 aryl group, a substituted or unsubstituted C3 to C30 heteroaryl group, or a combination thereof.