[css-content-3] Define computed value of 'content: normal' for ::placeholder and ::file-selector-button. w3c#6124

Author: fantasai

css-content-3/Overview.bs

@@ -130,7 +130,11 @@ Inserting and replacing content with the 'content' property</h2>
 
 			For ''::before'' and ''::after'', this computes to ''content/none''.
 
-			For ''::marker'', this computes to itself (''content/normal'').
+			For ''::marker'',
+			''::placeholder'',
+			and
+			''::file-selector-button'',
+			this computes to itself (''content/normal'').
 
 		<dt><dfn>none</dfn>
 		<dd>