Css custom attributes not working

Can someone help me understand why the code from the documentation is not working for me. I'm try to add custom attributes to my css load (so that I can bring in bootstrap) but custom attributes aren't working.

My code:
{% do assets.addCss('theme://css/styles.css',{'group':'mygroup'}) %} {{ assets.css('mygroup',{'rel':'alternate'}) }}

Is rendering as:
<link href="/user/themes/my-theme-name/css/styles.css" type="text/css" rel="stylesheet">

So... obviously not picking up my custom attribute. Here's the example from the documentation which is not working for me: https://learn.getgrav.org/16/themes/asset-manager#change-attribute-of-the-rendered-css-js-assets

Thanks.

Thank you. That got my attributes working.