flexbox.less provides universal support for 2009/2011 draft and standard versions of the flexbox specification.

flexbox.less

// flexbox.less
// https://gist.github.com/stephen-james/6493651

.flexbox(@inline : false) when (@inline)
{
  // support the 2009 Draft
  display : -webkit-inline-box;
  display : -moz-inline-box;
  display : inline-box;

  // support the 2011 Draft
  display : -ms-inline-flexbox;
  display : inline-flexbox;

  // support current (as at 2013)
  display : -webkit-inline-flex;
  display : inline-flex;
}

.flexbox(@inline : false) when (@inline = false)
{
  // support the 2009 Draft
  display : -webkit-box;
  display : -moz-box;
  display : box;

  // support the 2011 Draft
  display : -ms-flexbox;
  display : flexbox;

  // support current (as at 2013)
  display : -webkit-flex;
  display : -webkit-flex;
  display : flex;
}

.flexbox-align-items(@alignment) when (@alignment = center), (@alignment = baseline), (@alignment = stretch)
{
  // support the 2009 Draft
  -webkit-box-align : @alignment;
     -moz-box-align : @alignment;
          box-align : @alignment;

  // support the 2011 Draft
  -ms-flex-align : @alignment;

  // support current (as at 2013)
  -webkit-align-items : @alignment;
     -moz-align-items : @alignment;
      -ms-align-items : @alignment;
          align-items : @alignment;
}

.flexbox-align-items(@alignment) when (@alignment = start), (@alignment = flex-start)
{
  // support the 2009 Draft
  -webkit-box-align : start;
  -moz-box-align : start;
  box-align : start;

  // support the 2011 Draft
  -ms-flex-align : start;

  // support current (as at 2013)
  -webkit-align-items : flex-start;
  -moz-align-items : flex-start;
  -ms-align-items : flex-start;
  align-items : flex-start;
}

.flexbox-align-items(@alignment) when (@alignment = end), (@alignment = flex-end)
{
  // support the 2009 Draft
  -webkit-box-align : end;
  -moz-box-align : end;
  box-align : end;

  // support the 2011 Draft
  -ms-flex-align : end;

  // support current (as at 2013)
  -webkit-align-items : flex-end;
  -moz-align-items : flex-end;
  -ms-align-items : flex-end;
  align-items : flex-end;
}

.flexbox-justify-content(@packmethod) when (@packmethod = center)
{
  // support the 2009 Draft
  -webkit-box-pack: center;
  -moz-box-pack: center;
  box-pack: center;

  // support the 2011 Draft
  -ms-flex-pack: center;

  // support current (as at 2013)
  -webkit-justify-content: center;
  -moz-justify-content : center;
  -ms-justify-content : center;
  justify-content: center;
}

.flexbox-justify-content(@packmethod) when (@packmethod = start), (@packmethod = flex-start)
{
  // support the 2009 Draft
  -webkit-box-pack: start;
  -moz-box-pack: start;
  box-pack: start;

  // support the 2011 Draft
  -ms-flex-pack: start;

  // support current (as at 2013)
  -webkit-justify-content: flex-start;
  -moz-justify-content : flex-start;
  -ms-justify-content : flex-start;
  justify-content: flex-start;
}

.flexbox-justify-content(@packmethod) when (@packmethod = end), (@packmethod = flex-end)
{
  // support the 2009 Draft
  -webkit-box-pack: end;
  -moz-box-pack: end;
  box-pack: end;

  // support the 2011 Draft
  -ms-flex-pack: end;

  // support current (as at 2013)
  -webkit-justify-content: flex-end;
  -moz-justify-content : flex-end;
  -ms-justify-content : flex-end;
  justify-content: flex-end;
}

.flexbox-justify-content(@packmethod) when (@packmethod = justify), (@packmethod = space-between)
{
  // support the 2009 Draft
  -webkit-box-pack: justify;
  -moz-box-pack: justify;
  box-pack: justify;

  // support the 2011 Draft
  -ms-flex-pack: justify;

  // support current (as at 2013)
  -webkit-justify-content: space-between;
  -moz-justify-content : space-between;
  -ms-justify-content : space-between;
  justify-content: space-between;
}

.flexbox-justify-content(@packmethod) when (@packmethod = distribute), (@packmethod = space-around)
{
  // support the 2009 Draft
  -webkit-box-pack : distribute;
  -moz-box-pack : distribute;
  box-pack : distribute;

  // support the 2011 Draft
  -ms-flex-pack : distribute;

  // support current (as at 2013)
  -webkit-justify-content : space-around;
  -moz-justify-content : space-around;
  -ms-justify-content : space-around;
  justify-content : space-around;
}

.flexbox-align-self(@alignmethod) when (@alignmethod = auto), (@alignmethod = center), (@alignmethod = baseline), (@alignmethod = stretch)
{
  // not supported in the 2009 Draft

  // support the 2011 Draft
  -ms-flex-item-align : @alignmethod;

  // support current (as at 2013)
  -webkit-align-self : @alignmethod;
  -moz-align-self : @alignmethod;
  -ms-align-self : @alignmethod;
  align-self : @alignmethod;
}

.flexbox-align-self(@alignmethod) when (@alignmethod = start), (@alignmethod = flex-start)
{
  // not supported in the 2009 Draft

  // support the 2011 Draft
  -ms-flex-item-align : start;

  // support current (as at 2013)
  -webkit-align-self : flex-start;
  -moz-align-self : flex-start;
  -ms-align-self : flex-start;
  align-self : flex-start;
}

.flexbox-align-self(@alignmethod) when (@alignmethod = end), (@alignmethod = flex-end)
{
  // not supported in the 2009 Draft

  // support the 2011 Draft
  -ms-flex-item-align : end;

  // support current (as at 2013)
  -webkit-align-self : flex-end;
  -moz-align-self : flex-end;
  -ms-align-self : flex-end;
  align-self : flex-end;
}

.flexbox-align-content(@alignmethod) when (@alignmethod = center), (@alignmethod = stretch)
{
  // not supported in the 2009 Draft

  // support the 2011 Draft
  -ms-flex-line-pack : @alignmethod;

  // support current (as at 2013)
  -webkit-align-content : @alignmethod;
  -moz-align-content : @alignmethod;
  -ms-align-content : @alignmethod;
  align-content : @alignmethod;
}

.flexbox-align-content(@alignmethod) when (@alignmethod = start), (@alignmethod = flex-start)
{
  // not supported in the 2009 Draft

  // support the 2011 Draft
  -ms-flex-line-pack : start;

  // support current (as at 2013)
  -webkit-align-content : flex-start;
  -moz-align-content : flex-start;
  -ms-align-content : flex-start;
  align-content : flex-start;
}

.flexbox-align-content(@alignmethod) when (@alignmethod = end), (@alignmethod = flex-end)
{
  // not supported in the 2009 Draft

  // support the 2011 Draft
  -ms-flex-line-pack : end;

  // support current (as at 2013)
  -webkit-align-content : flex-end;
  -moz-align-content : flex-end;
  -ms-align-content : flex-end;
  align-content : flex-end;
}

.flexbox-align-content(@alignmethod) when (@alignmethod = justify), (@alignmethod = space-between)
{
  // not supported in the 2009 Draft

  // support the 2011 Draft
  -ms-flex-line-pack : justify;

  // support current (as at 2013)
  -webkit-align-content : space-between;
  -moz-align-content : space-between;
  -ms-align-content : space-between;
  align-content : space-between;
}

.flexbox-align-content(@alignmethod) when (@alignmethod = distribute), (@alignmethod = space-around)
{
  // not supported in the 2009 Draft

  // support the 2011 Draft
  -ms-flex-line-pack : distribute;

  // support current (as at 2013)
  -webkit-align-content : space-around;
  -moz-align-content : space-around;
  -ms-align-content : space-around;
  align-content : space-around;
}

.flexbox-order(@number)
{
  // support the 2009 Draft
  -webkit-box-ordinal-group : @number;
  -moz-box-ordinal-group : @number;
  box-ordinal-group : @number;

  // support the 2011 Draft
  -ms-flex-order : @number;

  // support current (as at 2013)
  -webkit-order : @number;
  -moz-order : @number;
  -ms-order : @number;
  order : @number;
}

.flexbox-flex(@grow : 0, @shrink : 1, @basis : auto)
{
  // support the 2009 Draft
  -webkit-box-flex : @grow;
  -moz-box-flex : @grow;
  box-flex : @grow;

  // support the 2011 Draft
  -ms-flex : @grow;

  // support current (as at 2013)
  -webkit-flex : @grow @shrink @basis;
  -moz-flex : @grow @shrink @basis;
  -ms-flex : @grow @shrink @basis;
  flex : @grow @shrink @basis;
}

.flexbox-direction(@direction) when (@direction = row)
{
  // support the 2009 Draft
  -webkit-box-orient : horizontal;
  -moz-box-orient : horizontal;
  box-orient : horizontal;
  -webkit-box-direction : normal;
  -moz-box-direction : normal;
  box-direction : normal;

  // support the 2011 Draft
  -ms-flex-direction : row;

  // support current (as at 2013)
  -webkit-flex-direction : row;
  -moz-flex-direction : row;
  -ms-flex-direction : row;
  flex-direction : row;
}

.flexbox-direction(@direction) when (@direction = row-reverse)
{
  // support the 2009 Draft
  -webkit-box-orient : horizontal;
  -moz-box-orient : horizontal;
  box-orient : horizontal;
  -webkit-box-direction : reverse;
  -moz-box-direction : reverse;
  box-direction : reverse;

  // support the 2011 Draft
  -ms-flex-direction : row-reverse;

  // support current (as at 2013)
  -webkit-flex-direction : row-reverse;
  -moz-flex-direction : row-reverse;
  -ms-flex-direction : row-reverse;
  flex-direction : row-reverse;
}

.flexbox-direction(@direction) when (@direction = vertical), (@direction = column)
{
  // support the 2009 Draft
  -webkit-box-orient : vertical;
  -moz-box-orient : vertical;
  box-orient : vertical;
  -webkit-box-direction : normal;
  -moz-box-direction : normal;
  box-direction : normal;

  // support the 2011 Draft
  -ms-flex-direction : column;

  // support current (as at 2013)
  -webkit-flex-direction : column;
  -moz-flex-direction : column;
  -ms-flex-direction : column;
  flex-direction : column;
}

.flexbox-direction(@direction) when (@direction = column-reverse)
{
  // support the 2009 Draft
  -webkit-box-orient : vertical;
  -moz-box-orient : vertical;
  box-orient : vertical;
  -webkit-box-direction : reverse;
  -moz-box-direction : reverse;
  box-direction : reverse;

  // support the 2011 Draft
  -ms-flex-direction : column-reverse;

  // support current (as at 2013)
  -webkit-flex-direction : column-reverse;
  -moz-flex-direction : column-reverse;
  -ms-flex-direction : column-reverse;
  flex-direction : column-reverse;
}

.flexbox-wrap(@wrapping) when (@wrapping = single), (@wrapping = nowrap)
{
  // support the 2009 Draft
  -webkit-box-lines : single;
  -moz-box-lines : single;
  box-lines : single;

  // support the 2011 Draft
  -ms-flex-wrap : nowrap;

  // support current (as at 2013)
  -webkit-flex-wrap : nowrap;
  -moz-flex-wrap : nowrap;
  -ms-flex-wrap : nowrap;
  flex-wrap : nowrap;
}

.flexbox-wrap(@wrapping) when (@wrapping = multiple), (@wrapping = wrap)
{
  // support the 2009 Draft
  -webkit-box-lines : multiple;
  -moz-box-lines : multiple;
  box-lines : multiple;

  // support the 2011 Draft
  -ms-flex-wrap : wrap;

  // support current (as at 2013)
  -webkit-flex-wrap : wrap;
  -moz-flex-wrap : wrap;
  -ms-flex-wrap : wrap;
  flex-wrap : wrap;
}

.flexbox-wrap(@wrapping) when (@wrapping = wrap-reverse)
{
  // not supported in the 2009 Draft

  // support the 2011 Draft
  -ms-flex-wrap : wrap-reverse;

  // support current (as at 2013)
  -webkit-flex-wrap : wrap-reverse;
  -moz-flex-wrap : wrap-reverse;
  -ms-flex-wrap : wrap-reverse;
  flex-wrap : wrap-reverse;
}