[WCAG AAA] - Move H28 from sufficient to advisory in 3.1.4 Abbreviations SC (#4195)

Closes: #3019 

[H28](https://www.w3.org/WAI/WCAG21/Techniques/html/H28) is not a
sufficient technique, rather than an advisory one, since it would likely
fail 2.1.1

Actions: moved H28 from sufficient to advisory.

---------

Co-authored-by: Mike Gower <michael.gower@ca.ibm.com>
Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>

Author: 3 people

techniques/html/H28.html

@@ -18,6 +18,9 @@ <h2>When to Use</h2>
       <h2>Description</h2>
       <p>The objective of this technique is to provide expansions or definitions for
             abbreviations by using the <code class="language-html">abbr</code> element. It is always appropriate to use the <code class="language-html">abbr</code> element for any abbreviation, including acronyms and initialisms.</p>
+            <p>This technique has been altered from a sufficient technique to an advisory technique because it uses the <code>title</code> attribute of <code>abbr</code> to provide the expansion of the abbreviation needed to achieve a sufficient technique when used in combination with <a href="../general/G102">G102</a>. As noted in the <a href="https://html.spec.whatwg.org/multipage/dom.html#the-title-attribute"> specification</a>:</p>
+
+               <blockquote><p>Relying on the title attribute is currently discouraged as many user agents do not expose the attribute in an accessible manner as required by this specification (e.g., requiring a pointing device such as a mouse to cause a tooltip to appear, which excludes keyboard-only users and touch-only users, such as anyone with a modern phone or tablet).</p></blockquote>
    </section>
 
    <section id="examples">

understanding/20/abbreviations.html

@@ -176,12 +176,6 @@ <h4>Situation A: If the abbreviation has only one meaning within the Web page:
                         <a href="../Techniques/general/G55" class="general">Linking to definitions</a>
                      </li>
 
-                     <li>
-                        												               
-                        <a href="../Techniques/html/H28" class="html">Providing definitions for abbreviations by using the abbr and acronym elements</a>
-                        											             
-                     </li>
-                     
                      <li>
 
                         <a href="../Techniques/pdf/PDF8" class="pdf"></a>
@@ -225,12 +219,6 @@ <h4>Situation A: If the abbreviation has only one meaning within the Web page:
 
                      </li>
 
-                     <li>
-                        												               
-                        <a href="../Techniques/html/H28" class="html">Providing definitions for abbreviations by using the abbr and acronym elements</a>
-                        											             
-                     </li>
-                     
                      <li>
 
                         <a href="../Techniques/pdf/PDF8" class="pdf"></a>
@@ -270,12 +258,6 @@ <h4>Situation B: If the abbreviation means different things within the same Web
 
                      </li>
 
-                     <li>
-                        												               
-                        <a href="../Techniques/html/H28" class="html">Providing definitions for abbreviations by using the abbr and acronym elements</a>
-                        											             
-                     </li>
-                     
                      <li>
 
                         <a href="../Techniques/pdf/PDF8" class="pdf"></a>
@@ -295,8 +277,9 @@ <h4>Situation B: If the abbreviation means different things within the same Web
 
       <section id="advisory">
          <h3>Additional Techniques (Advisory) for Abbreviations</h3>
-         
-         
+         <ul>
+           <li><a href="../Techniques/html/H28" class="html">Providing definitions for abbreviations by using the abbr and acronym elements</a></li>
+         </ul>         
       </section>
 
       <section id="failure">