Emphasis is different from Latin. There is interest in codifying how emphasis is done typographically for various scripts including Japanese

[macnmm]

Should document how emphasis is done typographically for Japanese in horizontal and vertical text in various media and publishing conventions.

[xfq]

Is this issue for jlreq or jlreq-d? In jlreq, there's a note about this: https://w3c.github.io/jlreq/#h-note-130

[xfq]

See also clreq 3.1.1.2.5 and 2.2.4. Underlines are used for indicating proper nouns, not emphasis (see clreq 3.1.1.2.9).

See also elreq 2.7.