[css-values-4] Add note that font-relative units can trigger a font download w3c#3135

Author: fantasai

css-values-4/Overview.bs

@@ -1889,7 +1889,10 @@ Font-relative Lengths: the ''em'', ''rem'', ''ex'', ''rex'', ''cap'', ''rcap'',
 	to the initial values of the 'font' and 'line-height' properties.
 	Similarly, when specified in a document with no root element,
 	the [=root font-relative lengths=] are resolved
-	assuming the initial values of the 'font' and 'line-height' properties..
+	assuming the initial values of the 'font' and 'line-height' properties.
+
+	Note: Font-relative units such as 'ch' and 'ic' can trigger font downloads,
+	if a required font is not yet loaded.
 
 	Some user-agents allow users to apply additional restrictions to font sizes in a document,
 	such as setting minimum font sizes to ensure readability.