Fullwidth is not equivalent to em

The definition for em is the total height of the quadrate. Assuming CJK glyphs share the same height, then the width of a CJK wide character is 1 em if and only if the font uses exact square character cells for wide characters. Square character cells are no longer guaranteed for CJK characters now. More and more fonts are using slightly narrow rectangles (e.g. 980/1000 as the width) or slightly wide rectangles for heavier variants. In such cases, the CJK full-width is no longer 1 em, nor is the CJK half-width 1 en.

This raises a series of issues. Firstly, em and full-width should be properly distinguished, so are en and half-width. em and en are units dependent on height (regardless of CJK or non-CJK) and full-width and half-width are dependent on East Asian width which are not only defined for CJK characters and symbols.

Secondly, any non-CJK characters that are defined and described in em or en in Unicode should probably not be recommended to use in CJK context. For instance, the punctuator "dash" (破折號) should be two horizontal bars (U+2015) instead of two em dashes (U+2014) nor one two-em dash (U+2E3A), which is anyways an ellipse rather than a dash.

Besides the width problem, those characters are meant to follow the alphabetic baseline which is not the same as the ideographic baseline, causing within-cell vertical alignment problems. Sharing alphabetic punctuators with ideographic characters should never be the first choice for typesetting reasons.

Start by reviewing the issue's discussion of em, en, full-width, half-width, and CJK punctuation in the Chinese Text Layout Requirements. Compare the current definitions and dash recommendations with the stated typography concerns, then confirm that any proposed revision clearly distinguishes height-based units from East Asian character widths and addresses baseline alignment.