Notice: Undefined index: #tag when used with the "Advanced CSS/JS Aggregation" module

This is a workaround, but it'd be worthwhile checking to see what $element contains to see why those variables don't exist in case there's another bug (or API change) somewhere.

Interesting! I wonder how one can reproduce this. On which URLs is this happening? AFAICT there are only two possible ways to get this:

1. HTML responses with no CSS at all
2. HTML responses with only external CSS

If that can be confirmed, then this will be committed & released right away :)

Thanks for checking those things!

So indeed, advagg is doing this:

$elements = ['prefetch' => []];

in \Drupal\advagg\Asset\CssCollectionRenderer::render().

The solution in #2 works, but is not quite correct. I think this is correct.

Advagg just released version 2.2 to fix this: http://cgit.drupalcode.org/advagg/commit/?h=8.x-2.x&id=0396b6a799a09ea47....

Sadly, the fix is incomplete; in fact, it causes D8 to break entirely. Once #2859132: Error: Unsupported operand types in Drupal\advagg\Render\HtmlResponseAttachmentsProcessor->processAssetLibraries() (line 64) is in, this is really fixed. Once that's also fixed in Advagg, then this problem will have disappeared.

However, that being said, I now see that #8 was completely wrong. I even wonder now what I was thinking then :)

The code in http2_server_push should simply be more robust. So, here's the code to make that more robust.

#13 fixes the problem. But let's be super thorough so that we don't ever have to deal with this again. Let's make it really ensure all assumptions are met, and let's fix it not only in \Drupal\http2_server_push\Asset\CssCollectionRenderer but also in \Drupal\http2_server_push\Asset\JsCollectionRenderer.

+++ b/src/Asset/AssetHtmlTagRenderElementTrait.php
@@ -0,0 +1,69 @@
\ No newline at end of file

Fixing this nit.

Zero coding standards violations:

wimleers in ~/Work/d8 on 8.4.x*
$ phpcs modules/http2_server_push --standard="core/phpcs.xml.dist"