The problem is in [LiteSpeed plugin](https://cs.wordpress.org/plugins/litespeed-cache/) specifically in: ```html <script src="some/src/file.js" async></script> ``` Turns into ```html <script type="litespeed/javascript" src="some/src/file.js" =async></script> ``` It seems like there is problem with some filter from LiteSpeed Same bug reported here: https://wordpress.org/support/topic/element-async/ Solution here: https://www.litespeedtech.com/support/forum/threads/resolved-js-defer-problem.15564/