I also thought that this was related to vertical-align of inline elements.
But no, the space is still present with vertical-align:bottom.
The background-color of the inline element covers the descender space of characters like j, p, q etc.
The space is below the characters.
I'll update the code & attachment with these characters
Couldn't edit the question. Code with descender characters & vertical-align: bottom:
<div class="cont"><span class="inn">inline content y p q</span></div>
border: 2px solid #F00;
background-color: #999; vertical-align: bottom;