[css-conditional-values-1] Expand motivation a little bit

Author: LeaVerou

css-conditional-values-1/Overview.bs

@@ -19,15 +19,31 @@ Introduction {#intro}
 
 	<em>This section is not normative.</em>
 
-	Authors frequently need to set a property to different values based
-	on the relation between certain values.
+Note: This section is a stub and needs to be expanded.
 
-	For example, a web component may support several keyword-based custom properties
-	and may want to set several different property values based on each keyword value.
+Authors frequently need to set a property to different values based
+on the relation between certain values.
 
-	As another example, ...
 
-	Note: TODO expand motivation
+High level custom properties {#high-level-custom-properties}
+-----------------------------
+
+A web component may support several custom properties which
+do not contain a value fragment verbatim, but set several properties across multiple rules indirectly.
+For example, a `--size` property with values `small`, `medium`, `large`,
+or an `--alignment` property with values `horizontal` and `vertical`.
+
+
+Relation between units of the same type {#relation-between-units}
+---------------------------------------
+
+Author code often needs to branch based on the relation between different units of the same type.
+
+For example:
+
+* Comparing viewport and absolute units allows compact one-off viewport dimension media queries
+* Comparing font-relative units and absolute <<length>> units allows authors
+	to apply different styling for small and large font sizes, enhancing readability.
 
 Value Definitions {#values}
 ---------------------------
@@ -65,6 +81,9 @@ Logical comparisons: The <<condition>> type {#condition}
 	<dfn>&lt;comparison-operator></dfn> = [ '=' | '>=' | '>' | '<' | '<=' ]
 </pre>
 
+Note: A future version of this module may expand <<comparison-operand>>
+to complex types, such as colors.
+
 <<condition>> values are logical expressions that resolve to a <<boolean-constant>>
 by performing simple comparisons and following basic boolean operators.
 When using `and` or `or` operators, precedence must be enforced with parentheses.