Block elements, by default, have a maximally possible width

Block elements, by default, have a maximally possible width ↓

Two <div> are located sequentially. By default, because these are sectional elements, they occupy a maximal width (width: auto).

HTML code:

<div class="first">
    ...
</div>
<div class="second">
    ...
</div>

Live example ↓

First div.first

.first
{
border: 2px solid #33F;
}

Second div.second

.second
{
border: 2px solid #F00;
}