[css-text] Change line-end behavior of text-spacing-trim: space-first

See w3c#9736

Author: frivoal

css-text-4/Overview.bs

@@ -10497,7 +10497,7 @@ CJK Punctuation Spacing: the 'text-spacing-trim' property</h3>
 			Set <a>fullwidth opening punctuation</a> with full-width glyphs (spaced)
 			on the first line the [=block container=]
 			and each line after a [=forced line break=].
-			Otherwise as ''trim-auto''.
+			Otherwise as ''text-spacing/normal''.
 
 			<wpt>
 				text-spacing-trim/text-spacing-trim-001.html
@@ -10541,6 +10541,15 @@ CJK Punctuation Spacing: the 'text-spacing-trim' property</h3>
 				to freely switch among the various spacing/indentation styles
 				without needing to alter the content.
 				See [[#japanese-start-edges]] for examples.
+
+				Additionally, the behavior at the end of lines is aligned
+				with the ''text-spacing-trim/normal'' and ''trim-start'' values
+				rather than ''text-spacing-trim/trim-auto'',
+				in that it only trims the glyphs
+				if they do not otherwise fit prior to justification.
+				While improving the typography in fewer cases,
+				it is closer to the legacy behavior of ''space-all''
+				which reduces compatibility concerns.
 			</details>
 
 		<dt><dfn>trim-start</dfn>
@@ -10599,11 +10608,10 @@ CJK Punctuation Spacing: the 'text-spacing-trim' property</h3>
 				<td>yes
 			<tr>
 				<th>''trim-start''
-				<td>only if would not fit
+				<td rowspan=2>only if would not fit
 			<tr>
 				<th>''space-first''
 				<td>yes except on the first line
-				<td>yes
 			<tr>
 				<th>''trim-all''
 				<td colspan=4>yes