[css-text-decor-3] Add note about how text-underline-position: under doesn't always get 100% under all the glyphs. #1055

Author: fantasai

css-text-decor-3/Overview.bs

@@ -401,6 +401,7 @@ Text Underline Position: the 'text-underline-position' property</h3>
             src="images/underline-position-alphabetic.png" >
         <p class="caption">A typical “alphabetic” underline is positioned just below the alphabetic baseline
       </div>
+
     <dt><dfn id="underline-under" value for="text-underline-position">under</dfn></dt>
     <dd>The underline is positioned <i>under</i> the element's text content.
       In this case the underline usually does not cross the descenders.
@@ -423,12 +424,19 @@ Text Underline Position: the 'text-underline-position' property</h3>
           texts that use many subscripts.
         <pre>:root { text-underline-position: under; }</pre>
       </div>
+
+      Note: The ''text-decoration/under'' value does not guarantee
+      that the underline will not conflict with glyphs,
+      as some fonts have descenders or diacritics
+      that extend below the font’s descent metrics.
+
     <dt><dfn id="underline-left" value for="text-underline-position">left</dfn></dt>
     <dd>In vertical <a>typographic modes</a>, the underline is aligned as for
       ''text-underline-position/under'', except it is always aligned to the left edge of the text.
       If this causes the underline to be drawn on the "over" side of
       the text, then an overline also switches sides and is drawn on
       the "under" side.
+
     <dt><dfn id="underline-right" value for="text-underline-position">right</dfn></dt>
     <dd>In vertical <a>typographic modes</a>, the underline is aligned as for
       ''text-underline-position/under'', except it is always aligned to the right edge of the text.