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CSS has grown from a language for formatting documents into a robust language for designing web applications. Its syntax is easy to learn, making CSS a great entry point for those new to programming. Indeed, it's often the second language that developers learn, right behind HTML.

The simplicity of CSS is deceptive, however. It belies the complexity of the box model, stacking contexts, specificity, and the cascade. It's tough to develop interfaces that work across a variety of screen sizes and with an assortment of input mechanisms. CSS mastery lies in understanding these concepts and how to mitigate them.

Mastering CSS development also means learning how to work with new tools such as linters, optimizers, and preprocessors. Linters inspect your code for potential trouble spots. Preprocessors make writing and organizing CSS easier. Optimizers improve CSS quality, and reduce the number of bytes delivered to the browser. And of course, there's the question of CSS architecture: which selectors to use, how to modularize files, and how to prevent selector creep.