While treebuilding, Parsoid converts some nodes to html comments with json encoded values to try to prevent them from being fostered. However, other comments on the page, present in the input, may contain valid json (see T279182). If that json happened to have the expected property ("@type": "mw:..."), we might run into some trouble. For a similar reason, the tokenizer already protects data attributes on dom nodes from similar conflicts with data-mw/data-parsoid/etc
See WTUtils::fosterCommentData / WTUtils::reinsertFosterableContent
Introduced in 0d6c4aaf4e3621d576c7a15a84b243af71af793b (all released Parsoid versions).