[css-images-4] Copy definition of 'loading image' to Images 4. Make image() aware of it. Properly specify behavior of image() when it contains only a URL, and that URL is invalid/loading. #1984

Author: tabatkins

css-images-4/Overview.bs

@@ -63,11 +63,32 @@ The <<image>> value type denotes a 2D image. It can be a
 	including the 'background-image', 'list-style-image', 'cursor' properties [[!CSS21]]
 	(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> have special behavior in some contexts,
-	such as the ''image()'' notation.
+	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.
 
 Image File Formats {#image-file-formats}
 ----------------------------------------
@@ -216,10 +237,14 @@ Image Fallbacks and Annotations: the ''image()'' notation {#image-notation}
 ### Image Fallbacks ### {#image-fallbacks}
 
 	If both a URL and a <<color>> are specified in ''image()'',
-	then whenever the URL represents an <a>invalid image</a>,
+	then whenever the URL represents an [=invalid image=] or [=loading image=],
 	the ''image()'' function renders as if the URL were not specified at all;
 	it generates a solid-color image as specified in [[#color-images]].
 
+	If just a URL is specified (no <<color>>)
+	and it represents an [=invalid image=] or [=loading image=],
+	the ''image()'' function represents the same.
+
 	<div class='example'>
 		The fallback color can be used to ensure that text is still readable
 		even when the image fails to load.