ie6不支持多class的样式

styles:
.c1{color:red;}
.c2{color:blue;}
.c1.c2{color:purple;}

.r.b{color:brown;}
.r {color:red;}
.b {color:blue;}

p.a.c{color: orange;}
p.c{color:green;}
p.a{color:red;}

class="c1 c2", 为 purple,看似 ie6 下没问题

class="c1",为 red,正常

class="c2",应为 blue,但 ie6 下是 purple

class="r b",ie6 下为 blue,其余为 brown

class="a c",ie6 下为 red,其余为 orange