https://drafts.csswg.org/css-text-4/#fullwidth-collapsing

• Set fullwidth opening punctuation half-width if the previous character is a fullwidth opening punctuation, fullwidth middle dot punctuation, or ideographic space (U+3000). Else set it full-width.
• Set fullwidth closing punctuation half-width if the next character is a fullwidth closing punctuation, fullwidth middle dot punctuation, or ideographic space (U+3000). Else set it full-width.
• Set fullwidth closing punctuation followed by fullwidth opening punctuation each at "3/4-width", i.e. halfway between full-width and half-width.

I think the last item above, "3/4-width" rule, is not necessary and the first item should be changed to:

• Set fullwidth opening punctuation half-width if the previous character is a fullwidth opening punctuation, fullwidth middle dot punctuation, fullwidth closing punctuation, or ideographic space (U+3000). Else set it full-width.

The reasons are as follows:

• as mentioned in #535 (comment), it is not appropriate when we use half-width / full-width glyphs.
• According to JIS X 4051, Table 5, the half-width space between the closing punctuation and the opening punctuation is attached to the former (closing) punctuation. i.e., the closing punctuation should be set full-width and the opening punctuation should be set half-width. (This is unclear in JLREQ but is clear in JIS)
• Existing typesetting software such as InDesign support the above JIS X 4051 rule.

Example 9 table (Demonstration of adjacent-pairs punctuation trimming) should also be changed:

Closing—Opening 　　〕+（　　)　　(

to

Closing—Opening 　　〕+（　　〕(

BTW, the current stylesheet in css-text-4 cannot render well this table. I found the css-text-3 stylesheet has the following style:

    .char { border: 1px dotted gray; }
    .quarter { font-size: 25%; }
    tt[lang="ja"] { font-family: "MS Gothic", "Osaka", monospace }

I think this should be moved to css-text-4 stylesheet, with some fixing:

    .char { border: 1px dotted gray; }
    .quarter { font-size: 25%; }
    samp[lang="ja"] { font-family: "MS Gothic", "Osaka-Mono", monospace }

Note that regular "Osaka" font is not appropriate because its fullwidth punctuations are not really full width (very narrow!) in current macOS font environment. So I think "Osaka-Mono" is better.

The CSS Working Group just discussed text-spacing fullwidth punctuation collapsing, and agreed to the following resolutions:

• RESOLVED: Accept change matching JLReq's punctuation spacing, put a note into the spec about maybe needing more complex stuff, follow up with JLReq about the topic.