[css-content] add to content property syntax to allow alternative text for generated content

Author: dauwhe

css-content/Overview.bs

@@ -80,7 +80,7 @@ figure:not([alt]) { content: attr(href, url), contents; }
 
 <pre class='propdef'>
     Name: content
-    Value: [ [ <<uri>> ] ',' ]* [ normal | none | <<content-list>> ]
+    Value: [ [ <<uri>> ] ',' ]* [ normal | none | <<content-list>> ] [/ <<string>> ]?
     Initial: normal
     Applies To: all elements, ''::marker'',  and page margin boxes.
     Inherited: no
@@ -90,7 +90,9 @@ figure:not([alt]) { content: attr(href, url), contents; }
 </pre>
 
 
-The 'content' property dictates what is rendered inside the element or pseudo-element. It takes a comma separated list of URIs, followed by a space separated list of tokens. If there are multiple URIs provided, then each is tried in turn until a value which is both available and supported is found. The last value is used as a fallback if the others fail.
+The 'content' property dictates what is rendered inside the element or pseudo-element. It takes a comma separated list of URIs, followed by a space separated list of tokens, followed by an optional slash and string indicating alternative text.  If there are multiple URIs provided, then each is tried in turn until a value which is both available and supported is found. The last value is used as a fallback if the others fail.
+
+
 
 
 <dl>