To Do List

Css-3 animations http://www.w3.org/TR/css3-animations/ There is no test suite. (http://www.root.cz/clanky/novinky-v-css3-animace/)
Css-3 namespaces - they are part css-3 selectors specification.
Css-3 paged media http://www.w3.org/TR/css3-page/
Non identifier element names in selectors.
Import keyword.
Fonts property declaration deserves another look. Does whitespaces distribution influences fonts?
Variables are still partially open - lazy loading is missing and they have not been tested with various types.
CSS hacks and quirks http://en.wikipedia.org/wiki/CSS_filter#.21important_quirks
Comments inside font-face, media and other less common elements can be lost during the translation. Review is needed.

Compile a list of tools to be used together with less. These discussions seems like a good start: http://www.quora.com/Which-is-better-Less-Compass-or-Sass and https://github.com/cloudhead/less.js/issues/1047.
eclipse plugin for syntax highlighting - maybe it already exists, find one.

Less.js does not support aspect-ratio in css3 media. E.g., this throws syntax error: @media (aspect-ratio: 59/80) {}.
Percentage as a Class Name - Less.js does not accept percentage as a class name. Less4j accepts it.
Less.js crashed on w3c selectors tests containing :not(:nth-xxx-xxxx(an+b)) and :not(:lang(xxx)). Those cases can be found in less.js-incompatible directory.
Less.js does not support floats with unit identifier e.g., this is illegal: 0.4cm