[css-anchor-1] Change anchor-scroll's initial value to default (w3c#8846)

* Change anchor-scroll's initial value to default

* Fix note indentation

Author: xiaochengh

css-anchor-position-1/Overview.bs

@@ -449,7 +449,7 @@ Taking Scroll Into Account: the 'anchor-scroll' property {#scroll}
 <pre class=propdef>
 Name: anchor-scroll
 Value: none | default | <<anchor-element>>
-Initial: none
+Initial: default
 Inherited: no
 Applies to: [=absolutely-positioned=] elements
 Animation Type: discrete
@@ -489,6 +489,11 @@ Its values are:
 		which will be compensated for in positioning and fallback.
 </dl>
 
+Note: When the element uses 'anchor-default'
+or has an [=implicit anchor element=],
+authors can often avoid explicitly setting an 'anchor-scroll' value
+because the initial value is ''anchor-scroll/default''.
+
 <div algorithm="compensate for scroll">
 	If 'anchor-scroll' is not ''anchor-scroll/none''
 	on an [=absolutely-positioned=] element |query el|,