I need to use the https://polyfill.io/v2/docs/ library for a Magento 2 project, so I called it in : htdocs/app/design/frontend/my_theme/default/Magento_Theme/layout/default_head_blocks.xml
With this code :
<script src="https://maxcdn.bhttps://cdn.polyfill.io/v2/polyfill.min.js" src_type="url" />
The library is loaded, but it's loaded AFTER Magento scripts and not BEFORE, so it doesn't work.