First (very) draft addition of adoptedStyleSheets

Author: mfreed7

css-cascade-5/Overview.bs

@@ -1035,6 +1035,7 @@ Cascade Sorting Order</h3>
 			For this purpose:
 
 			<ul>
+				<li>Style sheets are ordered as in <a href="https://www.w3.org/TR/cssom-1/#documentorshadowroot-final-css-style-sheets">final CSS style sheets</a>.
 				<li>Declarations from <a at-rule lt="@import">imported style sheets</a>
 					are ordered as if their style sheets were substituted in place of the ''@import'' rule.
 				<li>Declarations from style sheets independently linked by the originating document

cssom-1/Overview.bs

@@ -1088,19 +1088,26 @@ CSS Style Sheet Collections {#css-style-sheet-collections}
 Below various new concepts are defined that are associated with each
 {{DocumentOrShadowRoot}} object.
 
-Each {{DocumentOrShadowRoot}} has an associated list of zero or more
-<a>CSS style sheets</a>, named the
+Each {{DocumentOrShadowRoot}} has an associated list of zero or more <a>CSS style sheets</a>, named the
 <dfn export for=DocumentOrShadowRoot>document or shadow root CSS style sheets</dfn>. This is
-an ordered list that contains all
-<a>CSS style sheets</a> associated with the
-{{DocumentOrShadowRoot}}, in
-<a>tree order</a>, with
-<a>CSS style sheets</a> created from HTTP
-<code>Link</code> headers first, if any, in header
-order.
-
-To <dfn export>create a CSS style sheet</dfn>, run these
-steps:
+an ordered list that contains:
+<ol>
+  <li>Any <a>CSS style sheets</a> created from HTTP <code>Link</code> headers, in header order</li>
+  <li>Any <a>CSS style sheets</a> associated with the {{DocumentOrShadowRoot}}, in <a>tree order</a></li>
+</ol>
+
+Each {{DocumentOrShadowRoot}} has an associated list of zero or more <a>CSS style sheets</a>, named the
+<dfn export for=DocumentOrShadowRoot>final CSS style sheets</dfn>. This is
+an ordered list that contains:
+<ol>
+  <li>Any <a>CSS style sheets</a> created from HTTP <code>Link</code> headers, in header order</li>
+  <li>Any <a>CSS style sheets</a> associated with the {{DocumentOrShadowRoot}}, in <a>tree order</a></li>
+  <li>The contents of {{DocumentOrShadowRoot}}'s <a href="#dom-documentorshadowroot-adoptedstylesheets">adoptedStyleSheets</a>'
+  <a href="https://heycam.github.io/webidl/#observable-array-attribute-backing-list">backing list</a>,
+  in array order.</li>
+</ol>
+
+To <dfn export>create a CSS style sheet</dfn>, run these steps:
 
 <ol>
  <li>Create a new <a>CSS style sheet</a> object and set its
@@ -1285,12 +1292,20 @@ represented by the collection.
 <pre class=idl>
 partial interface mixin DocumentOrShadowRoot {
   [SameObject] readonly attribute StyleSheetList styleSheets;
+  attribute ObservableArray&lt;CSSStyleSheet> adoptedStyleSheets;
 };
 </pre>
 
 The <dfn attribute for=DocumentOrShadowRoot>styleSheets</dfn> attribute must return a {{StyleSheetList}} collection representing
 the <a>document or shadow root CSS style sheets</a>.
 
+The <a href="https://heycam.github.io/webidl/#observable-array-attribute-set-an-indexed-value">set an indexed value</a> algorithm
+for <dfn attribute for="DocumentOrShadowRoot"><code>adoptedStyleSheets</code></dfn>, given <var>value</var> and <var>index</var>, is the following:
+<ol>
+  <li>If <var>value</var>'s <a>constructed flag</a> is not set, or its <a>constructor document</a> is not equal to this
+  {{DocumentOrShadowRoot}}'s <a>node document</a>, throw a "{{NotAllowedError}}" {{DOMException}}.</li>
+</ol>
+
 Style Sheet Association {#style-sheet-association}
 --------------------------------------------------