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You're looking at a .css file — the stylesheet that controls how a web page looks. HTML defines what's on the page (headings, paragraphs, images). CSS defines how it looks (colours, fonts, layout, spacing, animations). Every styled website uses CSS. There is no alternative.
CSS stands for Cascading Style Sheets — "cascading" because styles can override each other based on specificity, source order, and inheritance. This cascade is both CSS's greatest strength (you can style an entire site from a single file) and its greatest source of frustration (figuring out why a button is blue when you told it to be green). Modern CSS includes flexbox, grid, custom properties (variables), animations, transitions, media queries for responsive design, and container queries.
Any text editor opens CSS. VS Code provides syntax highlighting, auto-completion, and colour previews. CSS changes are visible instantly in the browser (right-click → Inspect → edit styles live). For maintainable stylesheets in large projects, CSS preprocessors like SCSS add variables, nesting, and mixins — but they compile down to regular CSS in the end.