[css-overflow-3] Add overflow:overlay as a legacy value alias

See #8063

css-overflow-3/Overview.bs

@@ -455,7 +455,7 @@ Managing Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</h3
 
  <pre class="propdef shorthand">
  Name: overflow
- Value: [ visible | hidden | clip | scroll | auto ]{1,2}
+ Value: <'overflow-block'>{1,2}
  Initial: visible
  Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
  Inherited: no
@@ -542,6 +542,10 @@ Managing Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</h3
  is neither ''overflow/visible'' nor ''overflow/clip'' nor a combination thereof,
  it [=establishes an independent formatting context=] for its contents.
 
+ User agents must also support
+ the <dfn value for="overflow, overflow-x, overflow-y, overflow-block, overflow-inline">overlay</dfn> keyword
+ as a [=legacy value alias=] of ''overflow/auto''.
+
 <h4 id="scroll-visibility">
 Interaction of 'visibility' and 'overflow'</h4>