In this scenario, we encounter two different situations:
1. Divs A & B are placed side by side within a container
2. Div A is centered in the same container
For reference, you can view an example here
.container_one, .container_two {width:200px; height:100px; background-color:red;}
.container_one .inner_a {width:100px; background-color:green; float:left;}
.container_one .inner_b {width:100px; background-color:blue; float:right;}
.container_two .inner_a{width:100px; margin:0 auto; background-color:green;}
<div class="container_one">
<div class="inner_a">
a
</div>
<div class="inner_b">
b
</div>
</div>
<br>
<div class="container_two">
<div class="inner_a">
a
</div>
</div>
If both containers have the same class (unlike the given example), and the number of inner divs varies between 1 or 2, it becomes challenging to create CSS that accommodates both situations seamlessly. Despite numerous attempts, achieving this has proven to be difficult.