Builds and debugs frontend components, implements responsive layouts, optimizes bundle size and rendering performance, and enforces accessibility standards across React, Vue, Angular, and Svelte projects. Use when the user asks to create or refactor UI components, fix CSS/HTML/JavaScript issues, set up state management, improve Core Web Vitals, configure Tailwind or design tokens, debug hydration or rendering problems, or implement keyboard navigation and ARIA patterns in a web app.