Highlight note re: default vh unit (mdn#20269)

files/en-us/web/css/length/index.md

@@ -81,7 +81,7 @@ The viewport-percentage length units are based on four different viewport sizes:
 - **Default**
   - : The default viewport size is defined by the browser. The behavior of the resulting viewport-percentage unit could be equivalent to the viewport-percentage unit based on the small viewport size, the large viewport size, an intermediate size between the two, or the dynamic viewport size.
 
-    For example, a browser might implement the default viewport-percentage unit for height (`vh`) that is equivalent to the large viewport-percentage height unit (`lvh`).
+    > **Note:** For example, a browser might implement the default viewport-percentage unit for height (`vh`) that is equivalent to the large viewport-percentage height unit (`lvh`).  If so, this could obscure content on a full-page display while the browser interface is expanded.
 
 Viewport-percentage lengths define `<length>` values in percentage relative to the size of the initial [containing block](/en-US/docs/Web/CSS/Containing_block), which in turn is based on either the size of the {{Glossary("viewport")}} or the page area, i.e., the visible portion of the document. When the height or width of the initial containing block is changed, the elements that are sized based on them are scaled accordingly. There is a viewport-percentage length unit variant corresponding to each of the viewport sizes, as described below.