I knew the general shape of this: DOM, CSSOM, render tree, layout, paint.
Start by defining the Critical Rendering Path as the sequence of steps the browser takes to convert HTML, CSS, and JavaScript into pixels. Then walk through each stage in order—DOM construction, CSSOM construction, render tree, layout, paint, and composite—highlighting key optimizations and trade-offs. Conclude by tying it back to performance metrics and how you would optimize the path in practice.
Pro tip: Emphasize that the Critical Rendering Path is not just about rendering but also about blocking resources; mention how you'd use tools like Lighthouse and Chrome DevTools to identify and eliminate render-blocking resources, and discuss the impact of modern techniques like server-side rendering and code splitting.
Explain that it's the sequence of steps the browser takes to render a page, from receiving bytes to painting pixels. Mention that it's critical because it directly affects first paint and time to interactive.
Describe how HTML is parsed into the DOM and CSS into the CSSOM. Note that both are blocking: HTML parsing can be blocked by scripts, and CSSOM construction blocks rendering.
Explain that the browser combines the DOM and CSSOM into a render tree, which contains only visible elements and their computed styles. Mention that elements with display: none are excluded.
Describe how the browser calculates the exact position and size of each element (layout/reflow) and then fills in pixels (paint). Note that layout is expensive and can be triggered by changes.
Explain that layers are composited together, often on the GPU. Discuss optimizations like minimizing critical resources, using async/defer, and leveraging will-change or transform for animations.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.