I was checking out the new Lightning Design System framework from Salesforce, and noticed there's a pattern of img tags inside of span tags. Reference link - https://www.lightningdesignsystem.com/components/images/
<span class="slds-avatar slds-avatar--x-small"> <img src="/assets/images/avatar2.jpg" alt="person name" /> </span>
Any ideas why they're using this pattern in LDS putting a class on an enclosing span img tag vs adding the class directly to img? The CSS doesn't appear to be dependent on a span selector.