How to defer CSS via layout xml?

Question

I'm trying to defer a CSS file through default_head_blocks.xml.

After reading through as much as possible on here - and across the internet - I came across an answer suggesting to add defer="defer". This appears to have worked well on my JS files, but not on CSS. Many of the responses to that solution also stated that they had issues. Could anyone here please help me understand the best way of going about deferring CSS through layout (default_head_blocks.css)? This is for a non-essential chat plugin we're using for customer service, which doesn't need to render right away.

Thank you!