[css-anchor-position-1] Add CSSOM interfaces (w3c#9005)

Author: xiaochengh

css-anchor-position-1/Overview.bs

@@ -1262,6 +1262,48 @@ with no additional effects.)
 </div>
 
 
+<!--
+Insert ASCII art here
+-->
+
+DOM Interfaces {#interfaces}
+==========
+
+The CSSPositionFallbackRule interface {#position-fallback-rule}
+-------------------------------------
+
+The {{CSSPositionFallbackRule}} interface represents
+the ''@position-fallback'' rule:
+
+<pre class='idl' export>
+[Exposed=Window]
+interface CSSPositionFallbackRule : CSSGroupingRule {
+  readonly attribute CSSOMString name;
+};
+</pre>
+
+Its <dfn attribute for=CSSPositionFallbackRule>name</dfn> attribute
+represents the name declared by the at-rule itself.
+
+
+
+The CSSTryRule interface {#position-fallback-rule}
+-------------------------------------
+
+The {{CSSTryRule}} interface represents a ''@try'' block
+declared in a ''@position-fallback'' rule:
+
+<pre class='idl' export>
+[Exposed=Window]
+interface CSSTryRule : CSSRule {
+  [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
+};
+</pre>
+
+Its <dfn attribute for=CSSTryRule>style</dfn> attribute
+represents the declarations declared by the ''@try'' block itself in the specified order.
+
+
 Security Considerations {#sec}
 =======================