The HTML encoding of this document contains several errors, some of which substantially affect the way it's read. This fixes one of those problems in Appendix II:

javascript ([ ...document.querySelectorAll("op") ]).reverse().forEach((op) => { let f = document.createDocumentFragment(); f.append(document.createTextNode("<OP>"), ...op.childNodes); op.parentElement.replaceChild(f, op); })

The problem show be apparent on what is, at the time of this writing, line 4437:

html <code>IF ?w THEN ?x<OP>?y ELSE ?z<OP>?y</code>

(The angle brackets around the occurrences of "OP" should be encoded as HTML entities. Because they aren't they end up getting parsed as HTML op elements (which isn't a thing) and screwing up the document tree.)