less

//.mixin (@a) when (lightness(@a) >= 50%) {
      //255/2=127.5//  background-color: black;//}//.mixin (@a) when (lightness(@a) < 50%) {
   //  background-color: white;//}//.mixin (@a) {
   //  color: @a;//}//.class1 {
    .mixin(#7e7e7e) }  //221  > 127.5  >50%  background-color: black;  7e7e7e  =  126//.class2 {
    .mixin(#808080) }  //85 <127.5  <50%   background-color: white;  808080 = 128//iscolor,isnumber.....判断值得类型//.mixin (@a) when (iscolor(@a)) {
      //255/2=127.5//  background-color: black;//}//.mixin (@a) when (isnumber(@a) ) {
   //  background-color: white;//  shuzi:shuzi;//}//.mixin (@a) {
   //  color: @a;//}//.class1 {
    .mixin(#7e7e7e) }  //background-color: black;//.class2 {
    .mixin(123) }  //background-color: white;//ispixel,ispercentage.....单位检查函数.mixin (@a) when (ispixel(@a)) {
     background-color: black;}.mixin (@a) when (ispercentage(@a) ) {
     background-color: white;}.mixin (@a) {
     width: @a;}.class1 {
    .mixin(960px) }  //background-color: black; width:960px.class2 {
    .mixin(95%) }  //background-color: white;width:95%

css

.class1 {
     background-color: black;  width: 960px;}.class2 {
     background-color: white;  width: 95%;}