[css-grid-1][masonry] Remove align/justify-tracks from baseline alignment

But see also #9530 for open questions.

Author: fantasai

css-grid-3/Overview.bs

@@ -521,62 +521,15 @@ Baseline Alignment in the Masonry Axis</h3>
 	and the [=grid container=]'s baseline is determined
 	the same as for a regular [=grid container=] in that axis.
 
-	[=Baseline alignment=] is supported also in the [=masonry axis=],
-	on the first and last item in each track
-	(but not on items "in the middle" of the track).
-	Only tracks with the 'align-tracks' / 'justify-tracks' values
-	''align-tracks/start'', ''align-tracks/end'' or ''align-tracks/stretch''
-	support baseline alignment.
-	There are four different sets of [=baseline-sharing groups=]:
+	[=Baseline alignment=] is not supported in the [=masonry axis=].
+	The first baseline set of the [=grid container=] in this axis
+	is generated from the [=alignment baseline=] of
+	the first [=grid item=] in the first occupied track,
+	and the last baseline set from the last [=grid item=] placed.
 
-	<ol>
-		<li>the first item in each ''align-tracks/start'' track
-			+ the first item in each ''align-tracks/stretch'' track
-		<li>the last item in each ''align-tracks/start'' track
-		<li>the first item in each ''align-tracks/end'' track
-		<li>the last item in each ''align-tracks/end'' track
-			+ the last item in each ''align-tracks/stretch'' track
-	</ol>
-
-	Each of those sets can have a [=first baseline set=] and a [=last baseline set=],
-	resulting in eight unique baseline sets in the [=masonry axis=].
-
-	ISSUE: specify how the grid container's first(last) baseline in the [=masonry axis=] is determined
-
-	<div class="example">
-		Here's an <a href="examples/masonry-axis-baseline-alignment-1.html">example</a>
-		illustrating all eight possibilities.
-		(The example uses two separate [=grid containers=] to illustrate
-		the first ("F") and last ("L") baseline sets
-		to avoid cluttering the illustration,
-		but it is possible to use all eight sets in the same container.)
-		The aligned baselines are indicated with red color.
-		Note that the tracks that are attached to the end side of the [=masonry box=]
-		adjust the padding (or margin in the case of 'align-self') on the end side,
-		whereas tracks attached to the start side adjust the start padding/margin.
-		(Only 'align-content' is used in this example, which adjusts the padding,
-		since it's easier to see the baseline adjustment.
-		'align-self' can also be used, or a mix of the two, as usual.)
-
-		<figure>
-			<img src="images/masonry-axis-baseline-alignment-1.png">
-			<figcaption>
-				The rendering of the example above.
-			</figcaption>
-		</figure>
-	</div>
-
-
-	ISSUE: this needs more details about edge cases, caveat about misalignment in stretch, etc
-
-	Advisement: Authors are advised to be careful with using
-	alignment values other than ''align-tracks/start''
-	when some items span more than one track,
-	because it's easy to cause items to unintentionally overlap in this case.
+	ISSUE: We could support baseline alignment in the first row. Do we want to?
 
-	ISSUE: maybe we can make stretch alignment (at least) smarter
-	so that we avoid overlapping spanning items
-	in a few more cases that would be useful to authors?
+	ISSUE: Should the last baseline come from the last lowest item placed instead?
 
 <h2 id="pagination">
 Fragmentation</h2>