Merge pull request #3351 from w3c/Issue3337-visual-pres-i18n

Note under visual presentation

guidelines/sc/20/visual-presentation.html

@@ -13,5 +13,7 @@ <h4>Visual Presentation</h4>
  <li>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.</li> 
  <li>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 <a>on a full-screen window</a>.</li> 
  </ul>
+
+<p class="note">The 'mechanism' for achieving the presentation can be provided by the user-agent, which can manipulate the content on behalf of the user. It is not a requirement for the author to provide the mechanism. Some writing systems have additional or different requirements, or do not customarily use features such as paragraph spacing. When a specific feature is inapplicable to a writing system, this criterion does not require a mechanism to achieve that feature for content authored in that writing system.</p>
 
 </section>