preload CSS (loadCSS)
Since you integrated defer and async for "includeJS" so well i would like to see the same feature for "includeCSS".
Somethink like the following:
includeCSS.file1 = /PATH/file.css
includeCSS.file1.preload = 1
Which generates a Link-Tag with rel="preload", an as="style" and onload="this.onload=null;this.rel='stylesheet'", like suggested here https://github.com/filamentgroup/loadCSS
The usage of <link rel="preload"> is supported by around 75% of the browsers (https://caniuse.com/#feat=link-rel-preload) and is highly recommended by Google (PagespeedInsights/Lighthouse).
In my opinion this should be integrated for single files and for concatenated files.