Add missing headings to 1.4.8 Visual Presentation (#4286)

This restores headings that existed in the [2.0 version of Understanding
WCAG](https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-visual-presentation.html#visual-audio-contrast-visual-presentation-techniques-head),
and that are [still reflected in
Quickref](https://www.w3.org/WAI/WCAG22/quickref/?showtechniques=148#visual-presentation).

These headings were seemingly lost the moment 1.4.8 was migrated to this
repo, although the `section` elements are still there. The sentence that
precedes these sections doesn't make much sense without the headings.

This PR adds content, but is essentially a bug fix, as the content
existed previously but was lost. This will be needed in order to avoid
data loss when regenerating `wcag.json`. (I am currently working towards
that, thus I stumbled across this issue.)

Co-authored-by: Mike Gower <mikegower@gmail.com>

Author: kfranqueiro

Diff for: understanding/20/visual-presentation.html

@@ -247,6 +247,7 @@ <h3>Sufficient Techniques for Visual Presentation</h3>
          </p>
 
          <section>
+            <h4>First Requirement: Techniques to ensure foreground and background colors can be selected by the user</h4>
             <ul>
                <li>
                   <a href="../Techniques/css/C23" class="css">Specifying text and background colors of secondary content such as banners, features
@@ -273,6 +274,7 @@ <h3>Sufficient Techniques for Visual Presentation</h3>
             </ul>
          </section>
          <section>
+            <h4>Second Requirement: Techniques to ensure width is no more than 80 characters or glyphs (40 if CJK)</h4>
             <ul>
                <li>
                   <a href="../Techniques/general/G204" class="general">Not interfering with the user agent's reflow of text as the viewing window is narrowed</a> OR
@@ -283,6 +285,7 @@ <h3>Sufficient Techniques for Visual Presentation</h3>
             </ul>
          </section>
          <section>
+            <h4>Third Requirement: Techniques to ensure text is not justified (aligned to both the left and the right margins)</h4>
             <ul>
                <li>
                   <a href="../Techniques/css/C19" class="css">Specifying alignment either to the left OR right in CSS</a> OR
@@ -296,6 +299,10 @@ <h3>Sufficient Techniques for Visual Presentation</h3>
             </ul>
          </section>
          <section>
+            <h4>
+              Fourth Requirement: Techniques to ensure line spacing (leading) is at least space-and-a-half within paragraphs,
+              and paragraph spacing is at least 1.5 times larger than the line spacing
+            </h4>
             <ul>
                <li>
                   <a href="../Techniques/general/G188" class="general">Providing a button on the page to increase line spaces and paragraph spaces</a> OR
@@ -306,6 +313,10 @@ <h3>Sufficient Techniques for Visual Presentation</h3>
             </ul>
          </section>
          <section>
+            <h4>
+              Fifth Requirement: Techniques to ensure text can be resized without assistive technology up to 200 percent in a way that
+              does not require the user to scroll horizontally to read a line of text on a full-screen window
+            </h4>
             <ul>
                <li>
                   <a href="../Techniques/general/G204" class="general">Not interfering with the user agent's reflow of text as the viewing window is narrowed</a> OR