Tailwind JIT

This page uses the TailwindCSS JIT engine. The new Just-in-Time engine we announced in March has replaced the classic engine in Tailwind CSS v3.0.

Migrating to the JIT engine

Just-In-Time: The Next Generation of Tailwind CSS (2021)

This paragraph uses a TailwindCSS Feature from the Just-in-Time Mode: Generate arbitrary styles without writing custom CSS.
The className="text-[#f00]" generates a class that is not part of the default TailwindCSS config and not a custom theme config. This is a specific ulitity class with a text-color: #f00. And for color-schema dark className="dark:text-[#ff0]"

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