The vulnerability stems from the attribute quoting logic in the serializer. The commit 9b8d8eb introduced a ternary quote_attr_values system and expanded the regex pattern to include legacy-sensitive characters. The pre-patch code used a boolean flag and a limited regex (quoteAttributeSpec), failing to properly escape attributes containing characters like U+000B or ` which could break attribute context in legacy browsers. The serialize method directly controls attribute serialization and was the focal point of the security fix.