lib/less/parser/parser.js
function () {
                var e, expressions = [], index = parserInput.i;

                do {
                    e = this.expression();
                    if (e) {
                        expressions.push(e);
                        if (!parserInput.$char(',')) { break; }
                    }
                } while (e);

                if (expressions.length > 0) {
                    return new(tree.Value)(expressions, index);
                }
            }
function () {
                var e, expressions = [], index = parserInput.i;

                do {
                    e = this.expression();
                    if (e) {
                        expressions.push(e);
                        if (!parserInput.$char(',')) { break; }
                    }
                } while (e);

                if (expressions.length > 0) {
                    return new(tree.Value)(expressions, index);
                }
            }
A Value is a comma-delimited list of Expressions  font-family: Baskerville, Georgia, serif;  In a Rule, a Value represents everything after the `:`, and before the `;`.