[css-text-4] Add space-first value per Murakami's proposal in w3c#2462

Author: fantasai

css-text-4/Overview.bs

@@ -926,7 +926,7 @@ Character Class Spacing: the 'text-spacing' property</h3>
 		<tr>
 			<th>Value:
 			<td>normal | none |
-					[ trim-start | space-start ] ||
+					[ trim-start | space-start | space-first ] ||
 					[ trim-end | space-end | allow-end ] ||
 					[ trim-adjacent | space-adjacent ] ||
 					no-compress ||
@@ -1004,6 +1004,54 @@ Character Class Spacing: the 'text-spacing' property</h3>
 			Set <a>fullwidth opening punctuation</a> with half-width glyphs (flush)
 			at the start of each line.
 
+		<dt><dfn>space-first</dfn>
+		<dd>
+			Behaves as ''space-start'' on the first line the block container
+			and each line after a <a>forced line break</a>
+			but as ''trim-start'' on all other lines.
+
+			<details class="note">
+				<summary>This value exists for UA compat requirements,
+				and is not recommended for general authoring use.</summary>
+				This value exists to improve formatting of existing Japanese ePUB content,
+				for which ''trim-start'' would have been appropriate typographically,
+				except that they are typeset
+				to expect the first line to be set as ''space-first''.
+
+				Specifically,
+				due to the lack of reliable 'hanging-punctuation' support across ePUB readers,
+				such content uses U+3000 ideographic space in place of 'text-indent',
+				but omits it when the paragraph begins with punctuation
+				that is desired to hang in the indent
+				in order to create the hanging punctuation effect.
+				Using ''trim-start'' on the first line
+				would thus trim away the effective indent in such content
+				and thus obscure that line's distinction
+				as the first line of a new paragraph.
+
+				Note that this ePUB typesetting practice
+				is not recommended for CSS in general
+				(i.e. where not dictated by compat):
+				authors should use 'hanging-punctuation' and 'text-indent'
+				to control paragraph formatting
+				rather than tweaking the text content of the document.
+				This preserves the text’s true semantics in the document source
+				and allows the style sheet designer
+				to freely switch among the various spacing/indentation styles
+				without needing to alter the content.
+				See [[#japanese-start-edges]] for examples.
+
+				UAs are encouraged to use this value
+				as part of their UA default style sheet for Japanese ePUB content:
+				to preserve the paragraph distinctions in such content
+				while applying ''trim-start'' behavior to wrapped lines
+				(which creates better optical alignment along the start edge
+				and helps emphasize paragraph breaks denoted by indentation).
+
+				ISSUE: Whether this value should also be part of the UA defaults for Web content
+				is <a href="https://github.com/w3c/csswg-drafts/issues/2462">currently under discussion</a>.
+			</details>
+
 		<dt><dfn>allow-end</dfn>
 		<dd>
 			Set <a>fullwidth closing punctuation</a> with half-width glyphs (flush)