Find a better way for setting the onload attribute

Related to your code comment:

// TODO Find a better way for setting the onload attribute:
// Due to Drupal escaping quotes inside an attribute,
// we need to set a dummy "data-onload-rel" attribute, only needed
// for the onload attribute.
// 'this.rel="stylesheet"' gets escaped into
// 'this.rel="stylesheet"'.

You might be able to look to the AdvAgg module for ideas on this.

This is an example of the resulting markup from the module:

<link rel='preload' href="path/to/mystylesheet.css" as='style' onload="this.rel='stylesheet'"  media="all" />
<noscript><link rel="stylesheet" href="path/to/mystylesheet.css" media="all" /></noscript>

As an aside, Filament Group is now recommending nulling the onload handler with something like this:

<link rel="preload" href="path/to/mystylesheet.css" as="style" onload="this.onload=null;this.rel='stylesheet'">