[css-fonts-4] Mark initial value of font-min-size as unitless

http://lists.w3.org/Archives/Public/www-style/2017Mar/0044.html

Author: litherum

css-fonts-4/Overview.bs

@@ -692,7 +692,7 @@ at least as dark as its predecessor. Only values between 1 - 999 are valid, and
 	<pre class="propdef">
 	Name: font-min-size
 	Value: <<absolute-size>> | <<relative-size>> | <<length-percentage>>
-	Initial: 0px
+	Initial: 0
 	Applies to: all elements
 	Inherited: yes
 	Percentages: refer to parent element's font size