Lexical scoping accesses vars in current context. Closures "remember" vars & funcs for later use, like a remote control controlling a lamp from anywhere. Powerful tool with simple concept.

The Virtual DOM is an in-memory representation of the real DOM, constantly observed by React. It updates only changed nodes, avoiding refreshing the entire page for a smoother user experience.

Px for fine adjustments, rem for accessibility & scalability, em for dynamic padding & margins. Understand the differences to write elegant & adaptable code.