2.1. Text Decoration Lines: the text-decoration-line property
https://drafts.csswg.org/css-text-decor-3/#text-decoration-line-property

This section should probably mention that the actual position of an underline or overline is determined by the current setting of text-underline-position. This is particularly relevant for vertical text, where the line may be drawn on either side of the vertical line, depending on the setting of that property, or in the case of auto, of the browser default (which may vary for ja vs zh).