React Performance: JS Perf — Batch DOM CSS. Keywords: batch dom css reflow. Platform: React/Next.js. Description: Group CSS changes via classes or cssText to minimize reflows. Do: Use class toggle or cssText. Don't: Change styles one property at a time. Good Example: element.classList.add('highlighted'). Bad Example: el.style.width='100px'; el.style.height='200px'. Severity: Medium.