lib/less/parser/parser.js
function () {
                var value;

                // http://jsperf.com/case-insensitive-regex-vs-strtolower-then-regex/18
                if (!parserInput.$re(/^opacity=/i)) { return; }
                value = parserInput.$re(/^\d+/);
                if (!value) {
                    value = expect(parsers.entities.variable, 'Could not parse alpha');
                    value = '@{' + value.name.slice(1) + '}';
                }
                expectChar(')');
                return new tree.Quoted('', 'alpha(opacity=' + value + ')');
            }
function () {
                var value;

                // http://jsperf.com/case-insensitive-regex-vs-strtolower-then-regex/18
                if (!parserInput.$re(/^opacity=/i)) { return; }
                value = parserInput.$re(/^\d+/);
                if (!value) {
                    value = expect(parsers.entities.variable, 'Could not parse alpha');
                    value = '@{' + value.name.slice(1) + '}';
                }
                expectChar(')');
                return new tree.Quoted('', 'alpha(opacity=' + value + ')');
            }
IE's alpha function  alpha(opacity=88)