[css-text-3] Add note explaining the purpose of space-discarding appendix. w3c#337

Author: fantasai

css-text-3/Overview.bs

@@ -5499,6 +5499,45 @@ Space-Discarding Unicode Characters</h2>
   Han, Hiragana, Katakana, or Yi script
   shall also be considered part of the [=space-discarding character set=].
 
+  <details class="note">
+    <summary>Wherefore this table of “space-discarding characters”?</summary>
+
+    The purpose of the [[#line-break-transform|segment break transformation rules]]
+    is to “unbreak” text that has been formatted
+    with extra white space for source code readability,
+    see [[#line-break-transform]].
+
+    In most cases, “unbreaking” a line of text requires joining them with a space,
+    but some writing systems don't use spaces
+    so such texts need to be joined without any space.
+    CSS uses the characters before and after to determine
+    whether to join lines with or without a space.
+
+    For simplicity and for ease of implementation,
+    the classification of characters as space-discarding or space-preserving
+    is done by Unicode code block.
+    Ideally, such a list would be maintained in [[UNICODE]],
+    but the Unicode Technical Committee has yet
+    to express any intention of taking on this task.
+    In the meantime, in the interest of bringing
+    more of the text-processing facilities of CSS and HTML
+    that are available to Western writing systems
+    to Eastern writing systems as well,
+    the CSSWG is maintaining this appendix
+    and refining the rules in [[#line-break-transform]],
+    and hopes that in the future,
+    once CSS has demonstrated its viability,
+    the Unicode Consortium will recognize the need for an “unbreaking” algorithm
+    and take over maintenance of such.
+
+    <!-- things that could use an unbreaking algorithm:
+      * HTML/CSS
+      * Markdown
+      * TeX
+      * text editors' “unbreak lines” commands
+    -->
+  </details>
+
 <h2 id="script-tagging" class="no-num">Appendix G.
 Tagging Content by Writing System</h2>