If // style comments are stripped out of .less files this issue can be closed.

If however they survive in the resulting CSS they're not really valid. They will work apparently but they're not entirely correct. We should be using /* comment */

See theme/bootstrapbase/less/moodle/buttons.less for example.

At the same time this needs to be documented at http://docs.moodle.org/dev/CSS_coding_style#Documentation_and_Comments