v1.6.0

Properties can be interpolated, e.g. @{prefix}-property: value;
a default function has been added only valid in mixin definitions to determine if no other mixins have been matched
Added a plugins option that allows specifying an array of visitors run on the less AST
Performance improvements that may result in approx 20-40% speed up
Javascript evaluations returning numbers can now be used in calculations/functions
fixed issue when adding colours, taking the alpha over 1 and breaking when used in colour functions
when adding together 2 colours with non zero alpha, the alpha will now be combined rather than added
the advanced colour functions no longer ignore transparency, they blend that too
Added --clean-option and cleancssOptions to allow passing in clean css options
rgba declarations are now always clamped e.g. rgba(-1,258,258, -1) becomes rgba(0, 255, 255, 0)
Fix possible issue with import reference not bringing in styles (may not be a bugfix, just a code tidy)
Fix some issues with urls() being prefixed twice and unquoted urls in mixins being processed each time they are called
Fixed error messages for undefined variables in javascript evaluation
Fixed line/column numbers from math errors

v1.5.1

Added source-map-URL option
Fixed a bug which meant the minimised 1.5.0 browser version was not wrapped, meaning it interfered with require js
Fixed a bug where the browser version assume port was specified
Added the ability to specify variables on the command line
Upgraded clean-css and fixed it from trying to import
correct a bug meaning imports weren't synchronous (syncImport option available for full synchronous behaviour)
better mixin matching behaviour with calling multiple classes e.g. .a.b.c;

v1.5.0

sourcemap support
support for import inline option to include css that you do NOT want less to parse e.g. @import (inline) "file.css";
better support for modifyVars (refresh styles with new variables, using a file cache), is now more resiliant
support for import reference option to reference external css, but not output it. Any mixin calls or extend's will be output.
support for guards on selectors (currently only if you have a single selector)
allow property merging through the +: syntax
Added min/max functions
Added length function and improved extract to work with comma seperated values
when using import multiple, sub imports are imported multiple times into final output
fix bad spaces between namespace operators
do not compress comment if it begins with an exclamation mark
Fix the saturate function to pass through when using the CSS syntax
Added svg-gradient function
Added no-js option to lessc (in browser, use javascriptEnabled: false) which disallows JavaScript in less files
switched from the little supported and buggy cssmin (previously ycssmin) to clean-css
support transparent as a color, but not convert between rgba(0, 0, 0, 0) and transparent
remove sys.puts calls to stop deprecation warnings in future node.js releases
Browser: added logLevel option to control logging (2 = everything, 1 = errors only, 0 = no logging)
Browser: added errorReporting option which can be "html" (default) or "console" or a function
Now uses grunt for building and testing
A few bug fixes for media queries, extends, scoping, compression and import once.

1.4.2

• if you don't pass a strict maths option, font size/line height options are output correctly again
• npmignore now include .gitattributes
• property names may include capital letters
• various windows path fixes (capital letters, multiple // in a path)

1.4.1

• fix syncImports and yui-compress option, as they were being ignored
• fixed several global variable leaks
• handle getting null or undefined passed as the options object

1.4.0

Notes relative to last stable release (1.3.3)

• support for :extend() in selectors (e.g. input:extend(.button) {}) and &:extend(); in ruleset (e.g. input { &:extend(.button all); })
• Process ./ when having relative paths
• add isunit function for mixin guards and non basic units
• extends recognise attributes
• exception errors extend the JavaScript Error
• remove es-5-shim as standard from the browser
• Fix path issues with windows/linux local paths
• strictMaths option added - maths is now only done inside brackets. This means font: statements, media queries and the calc function can use a simpler format without being escaped. Enable this with --strict-math=on in lessc and strictMath:true in JavaScript.
• units are calculated, e.g. 200cm+1m = 3m, 3px/1px = 3. strictUnits option added - If you use units inconsistently you will get an error. to detect these errors use --strict-units=on in lessc or strictUnits:true in JavaScript
• (~"@var") selector interpolation is removed. Use @{var} in selectors to have variable selectors
• default behaviour of import is to import each file once. @import-once has been removed.
• You can specify options on imports to force it to behave as css or less @import (less) "file.css" will process the file as less
• variables in mixins no longer 'leak' into their calling scope
• added data-uri function which will inline an image into the output css. If ieCompat option is true and file is too large, it will fallback to a url()
• significant bug fixes to our debug options
• other parameters can be used as defaults in mixins e.g. .a(@A, @b:@A)
• an error is shown if properties are used outside of a ruleset
• added extract function which picks a value out of a list, e.g. extract(12 13 14, 3) => 3
• added luma, hsvhue, hsvsaturation, hsvvalue functions
• added pow, pi, mod, tan, sin, cos, atan, asin, acos and sqrt math functions
• added convert function, e.g. convert(1rad, deg) => value in degrees
• lessc makes output directories if they don't exist
• lessc @import supports https and 301's
• lessc "-depends" option for lessc writes out the list of import files used in makefile format
• lessc "-lint" option just reports errors
• support for namespaces in attributes and selector interpolation in attributes
• other bug fixes