W3C Validation Error

Soliloquy does this on purpose so that the stylesheet is only loaded on pages where a slider is present. It works in all major browsers (even IE6), and I choose this performance consideration over validating. If you want to validate, you can dequeue the style and then target the page specifically and enqueue it on wp_enqueue_scripts for that particular page. Here is a post on how to dequeue Soliloquy styles: https://wpsmith.net/2012/plugins/dequeue-soliloquy-style/