CSS cascade merge not work.

[zh-impact]

If the style input like that:

h1,
h2,
h3,
h4,
h5,
h6 {
  margin: 0;
}
h1 {
  font-size: 24px;
}

The margin set will lost because it replaced by the new rule, the result like that:

The margin of h1 appears again.

Expected: the css rule should cascade merged correct.