:first-child not working as expected

Question

The h1:first-child selector means

Select the first child of its parent
if and only if it’s an h1 element.

The :first-child of the container here is the ul, and as such cannot satisfy h1:first-child.

There is CSS3’s :first-of-type for your case:

.detail_container h1:first-of-type
{
    color: blue;
}

But with browser compatibility woes and whatnot, you’re better off giving the first h1 a class, then targeting that class:

.detail_container h1.first
{
    color: blue;
}