HS: element → HTML via outerHTML (+1 test)

Adds an `outerHTML` getter to the mock `El` class. Builds `<tag attrs>inner</tag>`
by merging `.id` / `.className` (set as direct properties via host-set!) with
the `.attributes` bag, and falling back to `innerText` / `textContent` when
there are no children or innerHTML.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/hs-run-filtered.js

@@ -99,6 +99,31 @@ class El {
       if (!('_origDefault' in this)) { this.defaultValue = this.textContent; this._origDefault = true; }
     }
   }
+  get outerHTML() {
+    const tag = this.tagName.toLowerCase();
+    // Merge .id / .className (set directly) into attribute output.
+    const attrOrder = [];
+    const attrMap = {};
+    if (this.id) { attrMap['id'] = this.id; attrOrder.push('id'); }
+    if (this.className) { attrMap['class'] = this.className; attrOrder.push('class'); }
+    for (const k of Object.keys(this.attributes)) {
+      if (!(k in attrMap)) { attrMap[k] = this.attributes[k]; attrOrder.push(k); }
+      else attrMap[k] = this.attributes[k]; // prefer explicit attr value when set
+    }
+    const attrs = attrOrder.map(k => ` ${k}="${String(attrMap[k]).replace(/"/g, '"')}"`).join('');
+    if (VOID_TAGS.has(tag)) return `<${tag}${attrs}>`;
+    let inner = '';
+    if (this.children && this.children.length) {
+      inner = this.children.map(c => c.outerHTML || c.textContent || '').join('');
+    } else if (this.innerHTML) {
+      inner = this.innerHTML;
+    } else if (this.innerText !== undefined && this.innerText !== '') {
+      inner = this.innerText;
+    } else if (this.textContent) {
+      inner = this.textContent;
+    }
+    return `<${tag}${attrs}>${inner}</${tag}>`;
+  }
   get firstElementChild() { return this.children[0]||null; }
   get lastElementChild() { return this.children[this.children.length-1]||null; }
   get nextElementSibling() { if(!this.parentElement)return null; const i=this.parentElement.children.indexOf(this); return this.parentElement.children[i+1]||null; }