src/Asset/AssetHtmlTagRenderElementTrait.php | 69 ++++++++++++++++++++++++++++
 src/Asset/CssCollectionRenderer.php          |  8 +++-
 src/Asset/JsCollectionRenderer.php           |  8 +++-
 3 files changed, 83 insertions(+), 2 deletions(-)

diff --git a/src/Asset/AssetHtmlTagRenderElementTrait.php b/src/Asset/AssetHtmlTagRenderElementTrait.php
new file mode 100644
index 0000000..33f9c13
--- /dev/null
+++ b/src/Asset/AssetHtmlTagRenderElementTrait.php
@@ -0,0 +1,69 @@
+<?php
+
+namespace Drupal\http2_server_push\Asset;
+
+trait AssetHtmlTagRenderElementTrait {
+
+  /**
+   * Whether the render element is a <link rel=stylesheet>.
+   *
+   * @param array $element
+   *   A render element.
+   *
+   * @return bool
+   */
+  protected static function isLinkRelStylesheet(array $element) {
+    return
+      (isset($element['#type']) && $element['#type'] === 'html_tag')
+      &&
+      (isset($element['#tag']) && $element['#tag'] === 'link')
+      &&
+      (isset($element['#attributes']) && isset($element['#attributes']['rel']) && $element['#attributes']['rel'] === 'stylesheet');
+  }
+
+  /**
+   * Whether the render element is a <script>.
+   *
+   * @param array $element
+   *   A render element.
+   *
+   * @return bool
+   */
+  protected static function isScript(array $element) {
+    return
+      (isset($element['#type']) && $element['#type'] === 'html_tag')
+      &&
+      (isset($element['#tag']) && $element['#tag'] === 'script');
+  }
+
+  /**
+   * Whether the render element is unconditional.
+   *
+   * An unconditional element is not browser-specific, i.e. is not wrapped in a
+   * conditional comment via the '#browsers' property.
+   *
+   * @see \Drupal\Core\Render\Element\HtmlTag::preRenderConditionalComments
+   *
+   * @param array $element
+   *   A render element.
+   *
+   * @return bool
+   */
+  protected static function isUnconditional(array $element) {
+    return empty($element['#browsers']) || ($element['#browsers']['!IE'] === TRUE && $element['#browsers']['IE'] === TRUE);
+  }
+
+  /**
+   * Whether the given attribute on the render element has a root-relative URL.
+   *
+   * @param array $element
+   *   A render element.
+   *
+   * @return bool
+   */
+  protected static function hasRootRelativeUrl(array $element, $attribute_name) {
+    $attribute_value = $element['#attributes'][$attribute_name];
+    return $attribute_value[0] === '/' && $attribute_value[1] !== '/';
+  }
+
+}
\ No newline at end of file
diff --git a/src/Asset/CssCollectionRenderer.php b/src/Asset/CssCollectionRenderer.php
index 4edb360..ef4e7c1 100644
--- a/src/Asset/CssCollectionRenderer.php
+++ b/src/Asset/CssCollectionRenderer.php
@@ -13,6 +13,8 @@ use Symfony\Component\HttpFoundation\RequestStack;
  */
 class CssCollectionRenderer implements AssetCollectionRendererInterface {
 
+  use AssetHtmlTagRenderElementTrait;
+
   /**
    * The decorated CSS collection renderer.
    *
@@ -49,8 +51,12 @@ class CssCollectionRenderer implements AssetCollectionRendererInterface {
     $request = $this->requestStack->getCurrentRequest();
     $link_headers = $request->attributes->get('http2_server_push_link_headers', []);
     foreach ($elements as &$element) {
+      if (!static::isLinkRelStylesheet($element)) {
+        continue;
+      }
+
       // Locally served CSS files that are sent to all browsers can be pushed.
-      if ($element['#tag'] === 'link' && $element['#browsers']['!IE'] === TRUE && $element['#browsers']['IE'] === TRUE && $element['#attributes']['href'][0] === '/' && $element['#attributes']['href'][1] !== '/') {
+      if (isset($element['#attributes']['href']) && static::hasRootRelativeUrl($element, 'href') && static::isUnconditional($element)) {
         $link_header_value = '<' . $element['#attributes']['href'] . '>; rel=preload; as=style';
         $link_headers[] = $link_header_value;
 
diff --git a/src/Asset/JsCollectionRenderer.php b/src/Asset/JsCollectionRenderer.php
index c1583a9..f9525a1 100644
--- a/src/Asset/JsCollectionRenderer.php
+++ b/src/Asset/JsCollectionRenderer.php
@@ -13,6 +13,8 @@ use Symfony\Component\HttpFoundation\RequestStack;
  */
 class JsCollectionRenderer implements AssetCollectionRendererInterface {
 
+  use AssetHtmlTagRenderElementTrait;
+
   /**
    * The decorated JS collection renderer.
    *
@@ -49,8 +51,12 @@ class JsCollectionRenderer implements AssetCollectionRendererInterface {
     $request = $this->requestStack->getCurrentRequest();
     $link_headers = $request->attributes->get('http2_server_push_link_headers', []);
     foreach ($elements as &$element) {
+      if (!static::isScript($element)) {
+        continue;
+      }
+
       // Locally served JS files that are sent to all browsers can be pushed.
-      if (isset($element['#attributes']['src']) && $element['#attributes']['src'][0] === '/' && $element['#attributes']['src'][1] !== '/' && (empty($element['#browsers']) || ($element['#browsers']['!IE'] === TRUE && $element['#browsers']['IE'] === TRUE))) {
+      if (isset($element['#attributes']['src']) && static::hasRootRelativeUrl($element, 'src') && static::isUnconditional($element)) {
         $link_header_value = '<' . $element['#attributes']['src'] . '>; rel=preload; as=script';
         $link_headers[] = $link_header_value;