what depends.
If your logo should be clickable, include it in the HMTL. (Usebility)
If it is present for design purposes only, use CSS.
As a rule, it is better to define everything related to the appearance of the Website in CSS.
HTML:
<header> <div id="company_logo"></div> </header>
CSS
#company_logo{ width:50px; height:50px; background-image:url(); }
source share