Why JavaScript Blocks the HTML Parser — and How It Affects the Critical Rendering Path

JavaScript blocks HTML parsing because it can modify the DOM structure while the browser builds it, potentially causing synchronization issues. When browsers encounter a regular script tag, they pause parsing, fetch and execute the script, then resume to maintain consistency. However, modern attributes like `defer`, `async`, and `type="module"` allow non-blocking execution, dramatically improving load speeds while preserving visual correctness.