[css-ui-3] Make accessibility requirement stand out visually

Author: frivoal

css-ui/Overview.bs

@@ -272,7 +272,9 @@ The outline properties control the style of these dynamic outlines.
 
 The stacking of the rendering of these outlines is explicitly left up to implementations to provide a better user experience per platform. This supersedes the stacking of outlines as defined in CSS 2.1.
 
+<strong class="advisement">
 Keyboard users depend on outline on elements in the :focus state for interaction with the page, thus authors must not make the outline invisible on such elements without making sure an alternative highlighting mechanism is provided.
+</strong>
 
 The rendering of applying transforms to outlines is left explicitly undefined in CSS3-UI.
 

css-ui/Overview.html

@@ -748,7 +748,9 @@ <h2 class="heading settled" data-level="4" id="outline-props"><span class="secno
    <p>The stacking of the rendering of these outlines is explicitly left up to implementations to provide a better user experience per platform. This supersedes the stacking of outlines as defined in CSS 2.1.</p>
 
 
-   <p>Keyboard users depend on outline on elements in the :focus state for interaction with the page, thus authors must not make the outline invisible on such elements without making sure an alternative highlighting mechanism is provided.</p>
+   <p><strong class="advisement">
+Keyboard users depend on outline on elements in the :focus state for interaction with the page, thus authors must not make the outline invisible on such elements without making sure an alternative highlighting mechanism is provided.
+</strong></p>
 
 
    <p>The rendering of applying transforms to outlines is left explicitly undefined in CSS3-UI.</p>