The backend load is almost nothing, everything is done client side for rendering, on the fly generation of the CSS and more. There is no CSS in the document, it's all rendered as it is needed and modified as it's needed. This also allows for colours based on percentages and OS set colours or modes like light and dark without having both loaded in the page.
It also means rich embeds and more like videos, tweets, articles and formatting.
It also means rich embeds and more like videos, tweets, articles and formatting.