Add header search form and stub behavior

index.html

@@ -182,6 +182,37 @@
           </span>
         </a>
 
+        <form
+          id="headerSearchForm"
+          role="search"
+          class="header-search"
+        >
+          <button
+            type="button"
+            class="header-search__filter"
+            aria-label="Open search filters"
+          >
+            <span aria-hidden="true" class="inline-flex h-5 w-5 items-center justify-center">
+              <img
+                src="assets/svg/filter.svg"
+                alt=""
+                aria-hidden="true"
+                class="h-5 w-5"
+              />
+            </span>
+          </button>
+          <label for="headerSearchInput" class="sr-only">Search videos</label>
+          <input
+            id="headerSearchInput"
+            class="header-search__input"
+            type="search"
+            name="query"
+            placeholder="Search videos"
+            autocomplete="off"
+          />
+          <button type="submit" class="header-search__submit">Search</button>
+        </form>
+
         <!-- Buttons on the far right -->
 
         <!-- Header action buttons share the fade-in treatment for consistency. -->