<p>Let's say I need to implement domain model for StackOverflow. </p>

<p>If I am doing ORM, how can I define (and map) property for fetching "last comments" and other "last" things?
It looks to me like this should be reflected in the domain model.</p>

<p>Sometimes I might need "all comments" though...</p>

Only put
```
width: 100%
```
on the outermost div, and don't put a border on it.  If you do this, then the inner boxes will fill the space (assuming you haven't floated them or anything) since they're block elements, and you won't have to worry about borders adding to the total size.
If you really need the appearance of five solid single pixel nested borders, you can do something like this (with properly semantic names, hopefully):
```
```
<div class="one">
    <div class="two">
        <div class="three">
        etc.
        </div>
    </div>
</div>

<style>
.one {
    width: 100%;
}
.two {
    border: 1px solid red;
    padding: 1px;
    background: red;
}
.three {
    border: 1px solid red;
    background: white;
}
</style>
```
```
As you can see, you can fake the second border using padding and background colors on the second div (might even cut down on the total number of divs by doing this; just remember you can't pad the outmost div without screwing up your width).