According to specs, a Layer name in an @ import file should be written without quotes : @import url(bootstrap.css) layer(bootstrap); (example 27 and others)

However, browsers support seems different upon using quotes or not :

• @import url("...") layer(bootstrap); (without quotes) : not OK on Chrome, Firefox, Safari (only tested on Mac)
• @import url("...") layer("bootstrap"); (with quotes) : OK on Chrome, Firefox, Safari (only tested on Mac)

See a live example :

https://codepen.io/raphaelgoetter/pen/ExrJBgb?editors=1100