[css-text-3] Restore markup improvements in kana mapping table

I had reverted more than necessary, so we should keep the good/safe
parts.

Follow up to #3209

Author: frivoal

css-text-3/Overview.bs

@@ -353,7 +353,7 @@ Characters and Letters</h3>
           This value is typically used to typeset Latin letters and digits
           as if they were ideographic characters.
       <dt><dfn>full-size-kana</dfn></dt>
-        <dd>Converts all small Kana characters to normal Kana.
+        <dd>Converts all small Kana characters to the equivalent full-size Kana.
           This value is typically used for ruby annotation text,
           where authors may want all small Kana to be drawn as large Kana
           to compensate for legibility issues at the small font sizes typically used in ruby.
@@ -2652,18 +2652,22 @@ Tagging Content by Writing System</h2>
 
 <h2 id="small-kana" class=no-num>Appendix G.
 Small Kana Mappings</h2>
+<style>
+.pairs-table th {
+  text-align: center;
+}
+</style>
 
+  <div class="pairs-table">
   <table class=data>
-   <caption>Small Kana Map to Normal Kana</caption>
-   <colgroup span=1></colgroup>
-   <colgroup span=1></colgroup>
+   <caption>Small Kana Map to Full-size Kana</caption>
 
    <thead>
     <tr>
-     <th scope=col>small
-     <th scope=col>normal
+     <th scope=col>Small
+     <th scope=col>Full-size
 
-   <tbody>
+   <tbody title="Hiragana">
     <!-- Vowels (Hiragana) -->
     <tr>
      <td>&#x3041; U+3041
@@ -2763,7 +2767,7 @@ Small Kana Mappings</h2>
      <td>
     -->
 
-   <tbody>
+   <tbody title="Katakana">
     <!-- Vowels (Katakana) -->
     <tr>
      <td>&#x30A1; U+30A1
@@ -2960,7 +2964,7 @@ Small Kana Mappings</h2>
      <td>
     -->
 
-   <tbody>
+   <tbody title="Half-width Katakana">
     <!-- Vowels (hwid) -->
     <tr>
      <td>&#xFF67; U+FF67
@@ -3020,7 +3024,7 @@ Small Kana Mappings</h2>
      <td>&#xFF6E; U+FF6E
      <td>&#xFF96; U+FF96
   </table>
-
+  </div>
 
 <h2 id="priv-sec" class="no-num">
 Privacy and Security Considerations</h2>