Function in_css_layer 

pub fn in_css_layer(css: &str) -> String

Wrap generated CSS in CSS_LAYER.

Every whole-stylesheet emitter in the family ends with this call. Exposed rather than kept private because an app that assembles its own stylesheet out of this family’s pieces has to put it in the same layer: goingson builds tables.css in its own build.rs from makeover_webview::list, and those rules are as generated as the ones in layout.css.