[css-text-3] Add a note at the top about atypical/ambiguous writing system situations, point at appendix.

Author: fantasai

css-text-3/Overview.bs

@@ -191,7 +191,8 @@ Languages and Typesetting</h3>
     or content in a <a>document language</a> that does not have a language-tagging facility
     is considered to have an unknown <a>content language</a>.
 
-    Note: Authors can tag content using the global <code>lang</code> attribute in HTML
+    Note: Authors can declare the [=content language=]
+    using the global <code>lang</code> attribute in HTML
     or the universal <code>xml:lang</code> attribute in XML.
     See the <a href="https://html.spec.whatwg.org/multipage/dom.html#language">rules
     for determining the content language of an HTML element</a> in [[HTML]],
@@ -205,6 +206,40 @@ Languages and Typesetting</h3>
     this information can be explicit or implied.
     See the normative [[#script-tagging]].
 
+    Note: Some languages have more than one writing system tradition;
+    in other cases a language can be transliterated into a foreign writing system.
+    Authors should <a href="#script-tagging">subtag</a> such cases
+    so that the UA can adapt appropriately.
+
+<!--
+	<div class=example>
+		For example, Korean (<code>ko</code>) can be written in
+		Hangul (<code>-Hang</code>),
+		Hanja (<code>-Hani</code>),
+		or a combination (<code>-Kore</code>).
+		Historical documents written solely in Hanja
+		do not use word spaces and
+		are formatted more like modern Chinese than modern Korean.
+		In other words, for typographic purposes <code>ko-Hani</code>
+		behaves more like <code>zh-Hant</code>
+		than <code>ko</code> (<code>ko-Kore</code>).
+
+		As another example Japanese (<code>ja</code>) is typically written
+		in a combination (<code>-Japn</code>) of Hiragana (<code>-Hira</code>),
+		Katakana (<code>-Kana</code>), and Kanji (<code>-Hani</code>).
+		However, it can also be ”romanized” into Latin (<code>-Latn</code>)
+		for special purposes like language-learning textbooks,
+		in which case it should be formatted more like English than Japanese.
+
+		As a third example contemporary Mongolian is written in two scripts:
+		Cyrillic (<code>-Cyrl</code>, officially used in Mongolia)
+		and Mongolian (<code>-Mong</code>, more common in Inner Mongolia, part of China).
+		These have very different formatting requirements,
+		with Cyrillic behaving similar to Latin and Greek,
+		and Mongolian deriving from both Arabic and Chinese writing conventions.
+	</div>
+-->
+
 <h3 id="characters">
 Characters and Letters</h3>