<span>A</span> <span>V</span>
Causes a space between these letters, how is it calculated expertly? How to contrast
<span>A</span><span>V</span>
The space between the built-in elements is compressed (or converted, if it is something other than spaces) into one space and displayed.
The W3C specification contains additional information:
This layout may include a space between words (called an interword space), but conventions for an interword space range from script to script. For example, in Latin scripts, the interlayer space is usually displayed as ASCII space (   ).
 
Try using a class over a range that will force a second span to a new line and CSS as
.spanClass { display:block; }
span is an inline element, such as plain text, so it follows the same style rules. When you write on a new line, rendering puts a space.
A B
will become "AB" with a space between A and B. It will also happen with a strong or em