Render-blocking
Resources (primarily CSS and synchronous scripts) that must be loaded and processed before the browser can paint anything on screen. They "block rendering" and delay the first pixel — a main cause of slow perceived loading.
Why: CSS is render-blocking because the browser doesn't want to paint unstyled content (FOUC) — it waits until all (head) CSS is fetched and parsed. Synchronous <script> (without defer/async) are parser-blocking — they stop HTML parsing until they're fetched and run. Result: large or slowly delivered CSS/JS in the <head> delays First Contentful Paint and LCP. Optimization: inline critical CSS (the small bit needed for the first thing visible) and load the rest asynchronously; use defer/async on scripts; reduce and split CSS/JS; and preload important resources. The browser flags these in performance tools as "render-blocking resources". Minimizing them is one of the most effective performance measures. Related to critical rendering path and preload/prefetch.