Autonomous Custom Element v1

🎉 The element is now using Custom Elements V1 API!

⚠️ Unfortunately, due to lack of polyfill and browser support, we were forced to drop customized built-in approach. So it's no longer inert <template is="imported-template"> but <imported-template>

⚠️ Currently, it only works in native environments, as there is a bug in webcomponents.js#v1 Html Imports polyfill webcomponents/webcomponentsjs#872. If the bug will not get fixed soon, we will implement a workaround by ourselves.