[css-font-loading] Clarifying some behavior for css-connected fontfaces.

Author: tabatkins

css-font-loading/Overview.bs

@@ -12,7 +12,7 @@ Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
 Former Editor: John Daggett, Mozilla, jdaggett@mozilla.com
 Abstract: This CSS module describes events and interfaces used for dynamically loading font resources.
 Link Defaults: css-fonts-3 (descriptor) src, dom-core-ls (interface) Event, typedarray (interface) ArrayBuffer/ArrayBufferView
-Ignored Terms: domstring, eventinit, cssfontfacerule, eventtarget, workerglobalscope, document, canvasproxy, iterator, unsigned long, set, src, iterator behavior
+Ignored Terms: domstring, eventinit, cssfontfacerule, eventtarget, workerglobalscope, document, canvasproxy, iterator, unsigned long, set, src, iterator behavior, InvalidModificationError, add(), delete()
 </pre>
 
 <h2 id="introduction">
@@ -955,6 +955,18 @@ Interaction with CSS’s ''@font-face'' Rule</h3>
 	must be added or removed from the document's <a>font source</a>,
 	and maintain this ordering.
 
+	Any manually-added {{FontFace}} objects must be ordered <em>after</em>
+	the <a>CSS-connected</a> ones.
+
+	When a {{FontFaceSet}} object's {{FontFaceSet/add()}} method is called with a <a>CSS-connected</a> {{FontFace}} object,
+	if the object is already in the set,
+	the operation must be a no-op;
+	otherwise, the operation must do nothing,
+	and throw an {{InvalidModificationError}}.
+
+	When a {{FontFaceSet}} object's {{FontFaceSet/delete()}} method is called with a <a>CSS-connected</a> {{FontFace}} object,
+	the operation must be a no-op.
+
 	Note: Authors can still maintain references to a removed {{FontFace}},
 	even if it's been automatically removed from a <a>font source</a>.
 	As specified in [[#font-face-css-connection]], though,