CSS Property 'offset' is IDL attribute 'cssOffset'

Like 'float', we can't use the name 'offset' as that already has
another meaning. So, like 'cssFloat', we use 'cssOffset' instead.

Discussed in FXTF issue and on blink-dev.
w3c/fxtf-drafts#51 (comment)
https://groups.google.com/a/chromium.org/d/msg/blink-dev/_DPl-JG6bV8/HXkK-CwfEQAJ

Overview.bs

@@ -6390,6 +6390,9 @@ The <dfn>animation property name to IDL attribute name</dfn> algorithm for
 1.  If <var>property</var> refers to the CSS 'float' property,
     return the string "cssFloat".
 
+1.  If <var>property</var> refers to the CSS 'offset' property,
+    return the string "cssOffset".
+
 1.  Otherwise, return the result of applying the <a>CSS property to IDL
     attribute</a> algorithm [[!CSSOM]] to <var>property</var>.
 
@@ -6402,6 +6405,9 @@ The <dfn>IDL attribute name to animation property name</dfn> algorithm for
 1.  If <var>attribute</var> is the string "cssFloat", then return
     an animation property representing the CSS 'float' property.
 
+1.  If <var>attribute</var> is the string "cssOffset", then return
+    an animation property representing the CSS 'offset' property.
+
 1.  Otherwise, return the result of applying the <a>IDL attribute to CSS
     property</a> algorithm [[!CSSOM]] to <var>attribute</var>.