Less language is an extension of css and less4j compiles it into regular css. Less adds several dynamic features into css: variables, expressions, nested rules, and so on. Less was designed to be compatible with css and any correct css file is also correct less file.
lessc does not consider the following an error, where less4j does:
com.github.sommeri.less4j.Less4jException: Could not compile less. 1 error(s) occurred:
ERROR my.less 3512:2 missing EOF at ';' in stylesheet (which started at 3:4)
3511: height: auto;
3512: };
3513:
lessc does not consider the following an error, where less4j does:
com.github.sommeri.less4j.Less4jException: Could not compile less. 1 error(s) occurred: ERROR my.less 3512:2 missing EOF at ';' in stylesheet (which started at 3:4) 3511: height: auto; 3512: }; 3513: