[css-text-3] Advance width of a tab must be at least 0.5ch. w3c#2883

Author: fantasai

css-text-3/Overview.bs

@@ -753,6 +753,8 @@ Characters and Letters</h3>
       <li>If the tab size is zero, tabs are not rendered.
         Otherwise, each tab is rendered as a horizontal shift
         that lines up the start edge of the next glyph with the next <a>tab stop</a>.
+        If this distance is less than 0.5<a href="https://www.w3.org/TR/css-values-3/#ch">ch</a>,
+        then the subsequent <a>tab stop</a> is used instead.
         <dfn lt="tab stop">Tab stops</dfn> occur at points that are multiples of the tab size
         from the block's starting content edge.
         The tab size is given by the 'tab-size' property.