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.