I think that the spec mentions behavior of text-emphasis + ruby (see below) but doesn't say behavior of combination of text-decoration and text-emphasis.

If emphasis marks are applied to characters for which ruby is drawn in the same position as the emphasis mark, the emphasis marks are placed outside the ruby.
https://drafts.csswg.org/css-text-decor-3/#text-emphasis-position-property

From an aesthetic point of view, text-decoration and text-emphasis should be separated from each other.

Furthermore, I feel that we have to think more complex pattern such as mixing text-decoration, text-emphasis and ruby.

See also: https://bugzilla.mozilla.org/show_bug.cgi?id=1262792