[css-grid] Make grid items size per 10.3.3 unless alignment says to use fit-content, consistent with cross-axis of flex items. Fix w3c#394 per WG resolution.

Author: fantasai

css-align/Overview.bs

@@ -870,6 +870,9 @@ Inline/Main-Axis Alignment: the 'justify-self' property</h3>
 	while still respecting the constraints imposed by 'min-height'/'min-width'/'max-height'/'max-width'.
 	Unless otherwise specified, this value falls back to ''flex-start''.
 
+	All other values cause a 'width'/'height' of ''width/auto''
+	to be treated as ''fit-content''.
+
 	Issue: Should ''justify-self/stretch'' allow a fallback alignment,
 	like it does on 'align-content' and 'justify-content'?
 

css-grid/Overview.bs

@@ -1060,12 +1060,18 @@ Grid Items</h2>
 	Grid Item Sizing</h3>
 
 	A <a>grid item</a> is sized within the containing block defined by its <a>grid area</a>
-	similarly to an equivalent block-level box in an equivalently-sized containing block,
-	except that ''margin/auto'' margins and the <a>box alignment properties</a>
-	have special effects. (See [[#alignment]].)
+	using the 'width' calculation rules for non-replaced block boxes
+	as defined in <a href="https://www.w3.org/TR/CSS21/visudet.html#blockwidth">CSS2.1 &sect; 10.3.3</a>
+	(meaning ''width/auto'' values in either axis are
+	effectively sized to fill the remaining space).
+	However, the <a>box alignment properties</a> have special effects:
+	when 'align-self'/'justify-self' is neither ''justify-self/normal'' nor ''justify-self/stretch'',
+	an ''width/auto'' size for the <a>grid item</a> in that axis
+	is treated as ''width/fit-content'' instead of as the <a>fill-available size</a>.
+	See [[#alignment]].
 
 	Note: The ''min-width/auto'' value of 'min-width' and 'min-height'
-	behaves on <a>grid items</a> in the relevant axis
+	affects track sizing in the relevant axis
 	similar to the flexbox behavior on <a>flex items</a> in the <a>main axis</a>.
 	See [[#min-size-auto]].