[css-images-3] Specify 'loading image' as a separate concept, for #1984.

Author: tabatkins

css-images-3/Overview.bs

@@ -101,15 +101,33 @@ Image Values: the <<image>> type {#image-values}
 	including the 'background-image', 'list-style-image', 'cursor' properties [[!CSS2]]
 	(where it replaces the <<url>> component in the property's value).
 
-	In some cases, an image is invalid,
-	such as a <<url>> pointing to a resource that is not a valid image format.
+	In some cases an image is invalid,
+	such as a <<url>> pointing to a resource that is not a valid image format
+	or that has failed to load.
 	An <dfn export lt="invalid image|valid image">invalid image</dfn> is rendered as a solid-color ''transparent'' image with no intrinsic dimensions.
 	However, <a>invalid images</a> can trigger error-handling clauses
 	in some contexts.
 	For example, an <a>invalid image</a> in 'list-style-image'
 	is treated as ''list-style-type/none'',
 	allowing the 'list-style-type' to render in its place. [[CSS2]]
 
+	While an image is loading,
+	is is a <dfn export>loading image</dfn>.
+	[=Loading images=] are <em>not</em> [=invalid images=],
+	but have similar behavior:
+	they are rendered as a solid-color ''transparent'' image with no intrinsic dimensions,
+	and may trigger fallback rendering in contexts that offer it,
+	but must not trigger loading of fallback resources.
+
+	Partially-loaded images (whose [=intrinsic dimensions=] are known, but whose image data is not fully loaded)
+	may be either treated as [=loading images=]
+	or as loaded images rendered with partial data.
+	For example, a UA may render an interlaced GIF in place
+	as soon as its first pass of pixel data has loaded
+	or even as soon as the image header (which contains sizing data) has parsed
+	and refresh the rendering as more data loads;
+	or it may wait until the entire image has loaded before using it.
+
 <!--
    ██ ██     ██ ████████  ██       ██
   ██  ██     ██ ██     ██ ██        ██