2.1.11 [HTML] Section 9.4, Marking document changes: The INS and DEL elements
The INS and DEL elements must not contain block-level content when these elements behave as inline elements.
All Document Modes (All Versions)
Block-level elements are supported. Instead of being forced into inline mode, block-level elements remain as block-level elements and are permitted to inherit INS styling. INS is treated as a block-level element.