Kaltura stores important information in the rel attribute of the span tag. Kaltura's JS then picks up that information, and replaces the span with a proper video embed.
Here is what Kaltura HTML looks like:
<span class="kaltura-media" rel="1_9jvcbo3f::video"><img src="https://cdnsecakmi.kaltura.com/p/999999/sp/9999154100/thumbnail/entry_id/1_9jvcbo3f/version/100000" /></span></p>
In the assignments tool an instructor can respond to a student with embedded comments like this
This creates ins and del tags which are two valid semantic html tags.