2.2.9 [HTML] Section 9.4, Marking document changes: The INS and DEL elements


The specification states:

 User agents should render inserted and deleted text in ways that make the change 
 obvious. For instance, inserted text may appear in a special font, deleted text may 
 not be shown at all or be shown as struck-through or with special markings, etc.

All Document Modes (All Versions)

Inserted text, or text within an INS element, is displayed as underlined text. Deleted text, or text within a DEL element, is displayed as text with strikethrough applied.