[css-fonts-4] port Character handling issues from L3

Author: svgeesus

css-fonts-4/Overview.bs

@@ -3073,7 +3073,58 @@ Cluster matching</h3>
 <h3 id="char-handling-issues">
 Character handling issues</h3>
 
-Issue: Import from level 3
+
+	CSS font matching is always performed on
+	text runs containing Unicode characters [[!UNICODE]],
+	so documents using legacy encodings are assumed
+	to have been transcoded before matching fonts.
+	For fonts containing <a>character maps</a>
+	for both legacy encodings and Unicode,
+	the contents of the legacy encoding <a>character map</a>
+	must have no effect on the results
+	of the font matching process.
+
+	The font matching process does not assume
+	that text runs are in either normalized or denormalized form
+	(see [[CHARMOD-NORM]] for more details).
+	Fonts may only support precomposed forms
+	and not the decomposed sequence of base character plus combining marks.
+	Authors should always tailor their choice of fonts to their content,
+	including whether that content contains
+	normalized or denormalized character streams.
+
+	If a given character is a Private-Use Area Unicode codepoint,
+	user agents must only
+	match font families named in the 'font-family' list
+	that are not generic families.
+	If none of the families
+	named in the 'font-family' list
+	contain a glyph for that codepoint,
+	user agents must display some form of missing glyph symbol
+	for that character
+	rather than attempting <a>system font fallback</a>
+	for that codepoint.
+	When matching the replacement character U+FFFD,
+	user agents may skip the font matching process
+	and immediately display
+	some form of missing glyph symbol,
+	they are not required to display
+	the glyph from the font
+	that would be selected
+	by the font matching process.
+
+	In general,
+	the fonts for a given family
+	will all have the same
+	or similar <a>character maps</a>.
+	The process outlined here
+	is designed to handle even font families
+	containing faces with widely variant <a>character maps</a>.
+	However,
+	authors are cautioned that
+	the use of such families
+	can lead to unexpected results.
+
 
 <!-- start new list wrt Fonts 3, if needed
 <h3 id="font-matching-changes">