[css-fonts-4] import cluster matching from level 3

Author: svgeesus

css-fonts-4/Overview.bs

@@ -2991,7 +2991,84 @@ given font families in the 'font-family!!property' list
 <h3 id="cluster-matching">
 Cluster matching</h3>
 
-Issue: Import from level 3
+
+	When text contains characters such as combining marks,
+	ideally
+	the base character should be rendered
+	using the same font as the mark,
+	this assures proper placement of the mark.
+	For this reason,
+	the font matching algorithm for clusters
+	is more specialized
+	than the general case of matching a single character by itself.
+	For sequences containing variation selectors,
+	which indicate the precise glyph to be used for a given character,
+	user agents always attempt <a>system font fallback</a>
+	to find the appropriate glyph
+	before using the default glyph of the base character.
+
+	A sequence of codepoints containing combining mark
+	or other modifiers
+	is termed a grapheme cluster
+	(see [[CSS3TEXT]] and [[UAX29]] for a more complete description).
+	For a given cluster containing a base character,
+	<em>b</em> and a sequence of combining characters
+	<em>c1, c2&hellip;</em>, the entire cluster is matched using these steps:</p>
+
+	<ol>
+
+		<li>For each family in the font list,
+			a face is chosen using the style selection rules
+			defined in the previous section.
+		<ol>
+
+		<li>If all characters in the sequence <em>b + c1 + c2 &hellip;</em>
+		are completely supported by the font,
+		select this font for the sequence.</li>
+
+		<li>If a sequence of multiple codepoints
+			is canonically equivalent to a single character
+			and the font <a>supports</a> that character,
+			select this font for the sequence
+			and use the glyph associated with
+			the canonically equiavlent character
+			for the entire cluster.</li>
+
+		</ol>
+
+		<li>If no font was found in the font list in step 1:
+
+		<ol>
+
+		<li>If <em>c1</em> is a variation selector,
+			system fallback must be used
+			to find a font that <em title="support">supports</em>
+			the full sequence of <em>b + c1</em>.
+			If no font on the system <a>supports</a> the full sequence,
+			match the single character <em>b</em>
+			using the normal procedure for matching
+			single characters
+			and ignore the variation selector.
+			Note: a sequence with more than one variation selector
+			must be treated as an encoding error
+			and the trailing selectors must be ignored. [[!UNICODE]]</li>
+
+		<li>Otherwise, the user agent may optionally use
+			system font fallback
+			to match a font that <a>supports</a>
+			the entire cluster.</li>
+
+		</ol>
+
+		<li>If no font is found in step 2,
+			use the matching sequence
+			from step 1 to determine the longest sequence
+			that is completely <a>supported</a>
+			by a font in the font list
+			and attempt to match the remaining combining characters
+			separately using the rules for single characters.</li>
+
+	</ol>
 
 <h3 id="char-handling-issues">
 Character handling issues</h3>