Add conditional stylesheet CSS for internet explorer only
I Found the solution in the comments on the drupal documentation for drupal_add_css():

drupal_add_css(
  path_to_theme() . '/less/ie.less',
  array(
    'browsers' =&gt; array(
      'IE' =&gt; 'lte IE 9',
      '!IE' =&gt; FALSE
    )
  )
);


Using this resulted in the LESS file being compiled by the LESS pre-processor, and the style was included within &lt;!--[if lte IE 9]&gt; comment tags.