[css-text-3] Clarify behavior of preserved tabs at the end of the line

Closes #3869

Author: frivoal

css-text-3/Overview.bs

@@ -1184,9 +1184,9 @@ Text Processing</h3>
     <li><p>If 'white-space' is set to ''pre'', ''pre-wrap'', or ''break-spaces'',
       any sequence of spaces is treated as a sequence of non-breaking spaces.
       However, for ''pre-wrap'',
-      a <a>soft wrap opportunity</a> exists at the end of the sequence,
+      a <a>soft wrap opportunity</a> exists at the end of a sequence of spaces and/or tabs,
       while for ''break-spaces'',
-      a <a>soft wrap opportunity</a> exists after every space.
+      a <a>soft wrap opportunity</a> exists after every space and every tab.
       <wpt>
         overflow-wrap/overflow-wrap-break-word-004.html
         overflow-wrap/overflow-wrap-break-word-005.html
@@ -1540,8 +1540,9 @@ Text Processing</h3>
           white-space-processing-046.xht
           white-space-processing-047.xht
         </wpt>
-      <li>If there remains any sequence of <a>white space</a>
-        and/or [=other space separators=]
+      <li>If there remains any sequence of <a>white space</a>,
+        [=other space separators=],
+	and/or tabs
         at the end of a line:
         <ul>
           <li>If 'white-space' is set to ''pre-wrap'',
@@ -1584,7 +1585,7 @@ Text Processing</h3>
              Note: [=Hanging=] the white space rather than collapsing it
              allows users to see the space when selecting or editing text.
           <li>If 'white-space' is set to ''break-spaces'',
-             [=hanging=] or collapsing the advance width of spaces
+             [=hanging=] or collapsing the advance width of the spaces or tabs
              at the end of the line is not allowed;
              those that overflow must <a>wrap</a> to the next line.
 

css-text-3/test-coverage.html

@@ -160,7 +160,16 @@ <h3 id="the-white-space-processing-rules">4.1 <a href="https://drafts.csswg.org/
 
 <h4 id="phase-i-collapsing-and-transformation">4.1.1 <a href="https://drafts.csswg.org/css-text-3/#white-space-phase-1">Phase I: Collapsing and Transformation</a></h4>
 
-<p>Sufficient coverage. All parts of this section are well exercised.
+<p>Almost Sufficient coverage. Most parts of this section are well exercised.
+
+<p>Missing tests:
+
+<ul>
+	<li>
+		a soft wrap opportunity exists at the end of a sequence of spaces <strong>and/or tabs</strong>,
+		while for break-spaces, soft wrap opportunity exists after every space <strong>and every tab</strong>.
+</ul>
+
 
 <h4 id="segment-break-transformation-rules">4.1.2 <a href="https://drafts.csswg.org/css-text-3/#line-break-transform">Segment Break Transformation Rules</a></h4>
 
@@ -189,6 +198,7 @@ <h4 id="phase-ii-trimming-and-positioning">4.1.3 <a href="https://drafts.csswg.o
 
 <ul>
 	<li>See [UAX9] for rules on how U+0009 tabulation interacts with bidi.
+	<li>tabs hang with ''pre-wrap'' and wrap with ''break-spaces''
 </ul>
 
 <h3 id="tab-character-size-the-tab-size-property">4.2 <a href="https://drafts.csswg.org/css-text-3/#tab-size-property">Tab Character Size: the tab-size property</a></h3>