fix w3c#4166 Unicode emoji presentation sequence

Author: Chris Lilley

css-fonts-4/Overview.bs

@@ -5771,10 +5771,10 @@ However, 'font-variant-emoji' allows web authors to set a default presentation s
 which can replace the variation selectors.
 
 Only the code points <a href="http://www.unicode.org/emoji/charts/emoji-variants.html">listed</a> by Unicode
-as contributing to a Unicode Presentation Sequence
+as contributing to a Unicode emoji presentation sequence
 are affected by this property.
 Within this CSS specification,
-these characters are referred to as <dfn id="presentation-participating-code-points">Presentation Participating Code Points</dfn>.
+these characters are referred to as <dfn id="emoji-presentation-participating-code-points">Emoji Presentation Participating Code Points</dfn>.
 This property has no effect on any other characters.
 
 This property is expected to affect font fallback;