yaru.css is a css library attempting to recreate the Ubuntu Yaru style in a web format. it also works as it's own implementation of something like bootstrap with built in GTK style elements like Boxes and Popovers. This is a simple test page showing the different elements.
You may have noticed this is a different item. yaru.css has the "box-v" div class for stacking different "box-content-1"s ontop of eachother, as long as the box-v is wrapped inside a box-content-1 of it's own. If you do not want this centering, just use two individual "box-content-1" classes.