Do not collect declared values in shadow subtrees

Follow up to a57cee4.

The algorithm to match a selector against a tree requires considering elements
in subtrees when no scope is defined, but only a few pseudo-elements can match
them at the moment, and they cannot match elements in the selector tree. The
subtree elements do not need to be included in the candidate elements.

The root of the element owning the style sheet cannot be defined as a scoping
root to fix this, because its selectors would be interpreted as scoped: `html`
would be interpreted as `:scope html` and match nothing.

This commit explicitly set a flag to skip shadow subtrees for the moment, until
the reason for this default behavior is clarified.

Author: cdoublev

lib/match/selector.js

@@ -1096,7 +1096,7 @@ function findPreviousSibling({ previousElementSibling }, predicate) {
  * @yields {Element}
  */
 function* traverse(tree, options = {}, inclusive = options.scopes?.inclusive) {
-    const { scopes, excludeShadowTrees } = options
+    const { scopes, excludeSubtrees } = options
     if (scopes?.limits?.includes(tree)) {
         return
     }
@@ -1112,7 +1112,7 @@ function* traverse(tree, options = {}, inclusive = options.scopes?.inclusive) {
     for (const element of children) {
         yield* traverse(element, options, true)
     }
-    if (shadowRoot && !excludeShadowTrees) {
+    if (shadowRoot && !excludeSubtrees) {
         for (const element of shadowRoot.children) {
             yield* traverse(element, {}, true)
         }

lib/resolve.js

@@ -284,6 +284,7 @@ function getStyleSheetListDeclarations(property, element, list, context) {
  */
 function createContext(tree, origin) {
     return {
+        excludeSubtrees: true,
         layers: [],
         namespaces: new Map,
         origin,