CSS3: Cascading Style Sheets

CSS controls the visual presentation, formatting, and responsive layout of HTML documents.

The CSS Box Model:

Every element on a web page is rendered as a rectangular box comprising 4 layers (from inside out):

1. Content: The actual text, image, or video.
2. Padding: Transparent space between the content and the border.
3. Border: A border surrounding the padding.
4. Margin: Transparent space outside the border separating the element from neighbors.

Modern Layout: Flexbox

.container {
    display: flex;
    justify-content: space-between; /* Horizontal alignment */
    align-items: center;            /* Vertical alignment */
    flex-wrap: wrap;
}