A post from Josh Comeau’s blog, “The Surprising Truth About Pixels and Accessibility” –

The truth is, if you want to build the most-accessible product possible, you need to use both pixels and ems/rems. It’s not an either/or situation. There are circumstances where rems are more accessible, and other circumstances where pixels are more accessible.

This is one of the best breakdowns of px/em/rem you will ever find. Highly recommended reading.