Conforming implementations of ECMA 262 edition 2 must properly support Unicode version 2.0 and ISO/IEC 10646-1 with UCS-2 as the encoding form. There are various sub-clauses to that encoding requirement and implementers should build their systems around the specifications laid down therein. Edition 3 requires slightly different character coding support.
Conforming implementations may provide additional capabilities. In particular, functionality flagged under the 'Future Reserved Word' category is encouraged by ECMA.
A strictly conforming implementation is one that only provides the features outlined in the standard. Since ECMAScript only defines core functionality and absolutely none of the host environment, a strictly conforming implementation of ECMAScript would be of limited use.
A conforming implementation provides the behavior outlined in the standard with some allowable additional behavior to support the hosting environment.