Preventing "double" borders in CSS

If we're talking about elements that cannot be guaranteed to appear in any particular order (maybe 3 elements in one row, followed by a row with 2 elements, etc.), you want something that can be placed on every element in the collection. This solution should cover that:

.collection {
    /* these styles are optional here, you might not need/want them */
    margin-top: -1px;
    margin-left: -1px;
}

.collection .child {
    outline: 1px solid; /* use instead of border */
    margin-top: 1px;
    margin-left: 1px;
}

Note that outline doesn't work in older browsers (IE7 and earlier).

Alternately, you can stick with the borders and use negative margins:

.collection .child {
    margin-top: -1px;
    margin-left: -1px;
}

#divNumberOne { border-right: 0; }