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The browser engine consists of several key components:

• JavaScript engine — responsible for parsing and executing JS code. For example, V8 in Chrome, SpiderMonkey in Firefox. It compiles JS into machine code and optimizes its execution.

• Rendering engine (layout engine) — responsible for parsing HTML and CSS, building the DOM and CSSOM, calculating styles, layout (positioning of elements), and rendering the page. Examples: Blink in Chrome, Gecko in Firefox.

• Network layer — handles HTTP requests, loading resources (HTML, CSS, JS, images).

• Security — includes same-origin policy, sandboxing, CORS, protection against XSS and other attacks.

The engine's operation begins with loading HTML, constructing the DOM, while CSS and JS are loaded in parallel. After building the DOM and CSSOM, layout and paint occur. The JS engine can influence the DOM, so sometimes a re-layout is triggered.

A simplified lifecycle example:

1. Load HTML ->
2. Build DOM ->
3. Load and parse CSS ->
4. Build CSSOM ->
5. Execute JS, which may modify DOM ->
6. Layout (calculate sizes and positions) ->
7. Paint (rendering) ->
8. Composition (assembling layers for display)

Optimizations such as minimizing reflow and repaint, lazy loading, cache management, and asynchronous resource loading are also studied.