Along with your questions, I've taken your comments into consideration in preparing this answer.
First, your HTML was invalid. The list was nested improperly so I corrected that that in my answer.
In answer to your first question...
how to position the prices at the baseline
... absolute positioning will work and will not prevent your price card from adjusting to different browsers, platforms or devices. It will be as responsive as the container it is in. Of course, you should test your code to make sure it works as intended.
Note that for position: absolute
to work properly you must set the parent element to position: relative
. This is because absolute positioning will move the element – in this case the em
– relative to its closest positioned ancestor (which in this case should be the li
). If the absolutely positioned element doesn't find a positioned ancestor, it will position the element relative to the <body>
. So bottom line:
To absolutely position a child element, set the parent element to position: relative
.
Here's an example using your code.
DEMO

HTML
<!-- with corrections to improperly nested list -->
<div id="container">
<h2>DRINK MENU</h2>
<ul>
<li><span>Latte</span><em>2.79</em></li>
<li><span>Cappucino</span><em>2.99</em></li>
<li><span>Cafe Americano more text more text more text more text</span>
<em>1.80</em></li>
<li><span>Espresso</span><em>2.00</em></li>
<li><span>Carmel Macchiato more text more text more text more text</span>
<em>10.00</em></li>
</ul>
</div>
CSS
/* based on your original code */
#container {
width: 200px;
border: 15px solid #886633;
background-color: orange;
box-shadow: 4px 4px 8px rgba(0, 0, 0, .3);
padding: 5px;
}
h2 {
width: 99%;
border-bottom: 1px solid black;
margin: 0;
}
ul {
list-style: none;
padding: 5px;
margin: 0;
}
#container ul li {
font-size: 1em;
font-weight: bold;
border-bottom: 1px dashed blue;
position: relative;
}
span {
width: 100px;
display: inline-block;
}
em {
position: absolute;
bottom: 0;
right: 0;
}
In answer to your second question...
Just tell me why is relative position not working.
Actually, it's working fine. In the normal flow of things, it's positioned exactly where it belongs. Your descriptions are breaking to a new line because of the margin limitation you set in your span
.
That being said, the em
can still be positioned with position: relative
. Change the value from 0. Your prices will (as defined by your style rule) move up or down as a group, depending on whether you use positive or negative numbers.
Your CSS rule:
em {
float: right;
position: relative;
bottom: 0px;
/* test these individually:
bottom: 25px;
bottom: -25px;
right: 25px;
right: -25px */
}
For more about positioning see the position
article at MDN.