Hello @mikoMK, I appreciate your reply.
I looked over “From object to iframe (The <embed> and <object> elements)” of both the English and the Japanese. Only in the latter, I saw typemustmatch as an attribute of the object element.
<object data="mypdf.pdf" type="application/pdf"
width="800" height="1200" typemustmatch>
At least, this translation didn’t look old*. However, I couldn’t find the attribute on the element page, the global attributes page, and " The 26 Boolean Attributes of HTML".
I also found the attribute in the HTMLObjectElement page of the same language, but I couldn’t find it on the English version. This translation also didn’t look old*. typemustmatch in the translation has a link to the object element page of the same language (object#attr-typemustmatch), but I couldn’t find the attribute in the page*.
*All the translations I checked showed “Last modified: March 12, 2022”.