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 + ')');
            }
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)
IE s alpha function alpha ( opacity = 88 )