The iframe section states: > To solve this, we could create an HTML file that contains only the <svg> tag, and reference it on the site via an <iframe> Theref

The iframe section states: > To solve this, we could create an HTML file that contains only the <svg> tag, and reference it on the site via an <iframe> Therefore you can attach the stylesheet containing the styles for the button/checkbox/etc. in the head of that document. I don't know exactly how you would do this (w.r.t. propagating hover, etc. state) as I've used the embedded SVG element approach when I need styling.