BOSS uses different approach. If you want to have declarations of "button" in "button-primary" you can just write <button class="button button-primary">

BOSS uses different approach. If you want to have declarations of "button" in "button-primary" you can just write <button class="button button-primary"> But is you want to reuse some fancy image-border "button" and "input", so they may have nothing in common, except that image-border. You can create a mixin with desired declarations for all browser, and then use it in your code as simple as "fancy-image-border: url('image.png')". And all rulesets, that have this "fancy-image-border" will then get a list of declarations in place of this mixin. So you see, the syntax is pretty simple and matches CSS-syntax. You don't need @extends or smth else, you just have to know the name of mixin declaration.