I'm not 100% sure what you mean by "this", but I have a suspicion. The original CSS was:

#rendered-md h1:first-child,
#rendered-md h2:first-child,
#rendered-md h3:first-child,
#rendered-md h4:first-child,
#rendered-md ul:first-child,
#rendered-md ol:first-child,
#rendered-md table:first-child,
#rendered-md blockquote:first-child,
#rendered-md img:first-child,
#rendered-md p:first-child {
  margin-top: 0;
  padding-top: 0;
}

This covers all h1..p elements that are a first child of some element AND are also a descendant (in whatever degree) of #rendered-md. That's a bit too much. If it is replaced with:

#rendered-md > h1:first-child,
#rendered-md > h2:first-child,
#rendered-md > h3:first-child,
#rendered-md > h4:first-child,
#rendered-md > ul:first-child,
#rendered-md > ol:first-child,
#rendered-md > table:first-child,
#rendered-md > blockquote:first-child,
#rendered-md > img:first-child,
#rendered-md > p:first-child {
  margin-top: 0;
  padding-top: 0;
}

then it will cover all h1..p elements that are the first direct child of #rendered-md. That's exactly what we want, isn't it?