[css-values-5] Defined <fraction> more clearly

Author: SebastianZ

css-values-5/Overview.bs

@@ -127,19 +127,18 @@ Other Quantities</h2>
 -->
 
 <h3 id="fractions">
-Flexible lengths: the <<fraction>> type</h3>
-
-	Fraction values are denoted by <dfn id="flex-value">&lt;fraction></dfn>,
-	and indicate a value that is some fraction of another reference value.
-	A fraction is a <<dimension>>.
-	Its unit identifier is <dfn value for="<flex>" lt="fr|fr unit">fr</dfn>.
-
-	These values specify
-	<dfn export lt="flexible length">flexible lengths</dfn>.
-	Each length defines a portion of a given space
-	which is specified by the context the type is used in.
-
-	The allowed range of <<fraction>>> values
+Proportional Fractions: the <<fraction>> type</h3>
+
+	<dfn id=flex-fraction>Proportional fractions</dfn> are denoted by <dfn id="fr-value"><<fraction>></dfn>,
+	which is a <<dimension>> with the <dfn value for="<fraction>" lt="fr|fr unit">fr</dfn> unit.
+	They represent a fraction of a reference value
+	(typically a length)
+	as a proportion in relation to other [=flex fractions=] referencing that same value,
+	i.e. a value of ''<var>N</var>fr'' represents <var>N</var> / max(<var>sum of all ''fr'' values</var>, 1).
+	For example, leftover space in [=grid layout=]
+	is distributed to a ''1fr'' track and a ''2fr'' track in a 1:2 ratio.
+
+	The allowed range of <<fraction>> values
 	<em>always</em> excludes negative values,
 	in addition to any explicit ranges that might be specified.