Why CSS Still Matters for Performance
CSS is easy to overlook when tuning page speed, but it sits directly in the critical rendering path. The browser must construct both the DOM and the CSSOM before it can paint anything, and that means every stylesheet it encounters gets a high download priority.
Any time [the browser] encounters any external resource (CSS, JS, images, etc.) it will assign it a download priority and initiate its download. Priorities are important because some resources are critical to render a page (eg. main stylesheet and JS files) while others may be less important (like images or stylesheets for other media types).
Because stylesheets feed the CSSOM, they are inherently “blocking.” That is partly by design—nobody wants a flash of unstyled content—but the cost is real: smaller CSS means faster downloads, faster parsing, and faster application.
Architectural Trade-offs Worth Noting
Not all styling approaches carry the same weight. Atomic or utility-first stylesheets tend to be significantly smaller in total output, which gives them a built-in performance advantage. CSS-in-JS, on the other hand, shifts the cost around: you avoid loading CSS at the top of the page, but the JavaScript bundle grows to compensate. Whether that trade-off nets out in practice is still an open question—there isn’t a rigorous head-to-head study to settle it.
