[css-images-4][editorial] Mark as delta spec, fix w3c#5804

Author: svgeesus

css-images-4/Overview.bs

@@ -13,7 +13,9 @@ Editor: Lea Verou, Invited Expert, http://lea.verou.me/about, w3cid 52258
 Abstract: This module contains the features of CSS level 4 relating to the <<image>> type and replaced elements.
 	It includes and extends the functionality of CSS level 2 [[CSS2]] and in the previous level of this specification [[css-images-3]].
 	The main extensions compared to "CSS Images Module Level 3" [[css-images-3]] are several additions to the <<image>> type, such as the ''image()'' notation, the ''element()'' notation, and conic gradients.
+	This level is currently maintained as a diff spec over the level 3 module.
 
+!Delta Spec: yes
 Issue Tracking: Tracker http://www.w3.org/Style/CSS/Tracker/products/27
 Previous Version: https://www.w3.org/TR/2017/WD-css-images-4-20170413/
 Ignored Terms: <offset>, background positioning area, border image area, <meetorslice>, Map, <image>, invalid image, invalid images, concrete object size, linear-gradient(), radial-gradient(), default object size, CSS