We recommend you to read the blog post Reimagine Atomic CSS wrote by the creator of UnoCSS, Anthony Fu, to get a better understanding of the motivation behind UnoCSS.
How is UnoCSS Different from X?
UnoCSS is started by one of the Windi CSS's team members, with a lot of inspirations taken from the work we did in Windi CSS. While Windi CSS is no longer actively maintained (as of March 2023), you may consider UnoCSS as a "spiritual successor" of Windi CSS.
UnoCSS inherits Windi's on-demand natural, attributify mode, shortcuts, variant groups, compilation mode and a lot more. On top of that, UnoCSS is built ground up with the maximum extensibility and performance in mind, making us able to introduced new features like pure CSS icons, valueless attributify, tagify, web fonts, etc.
Mostly importantly, UnoCSS is extracted as an atomic-CSS engine, where all the features are optionally opt-in, and making it easy to create your own conventions, own design system, and own presets - with the combinations of the features you want.
Both Windi CSS and UnoCSS took a lot of inspirations from Tailwind CSS. Since UnoCSS is built ground up, we are able to have a great overview of how atomic CSS been designed with prior arts and abstract into a elegant and powerful API. With quite different design goals, its not really an apples-to-apples comparison with Tailwind CSS. But we will try to list a few differences:
Tailwind CSS is a PostCSS plugin, while UnoCSS is an isomorphic engine with a bunch of first-class integrations with build tools (including a PostCSS plugin). This means UnoCSS can be much more flexible to be used in different places (for example, CDN Runtime, which generates CSS on the fly) and have deep integrations with build tools to provide better HMR, performance, and developer experience (for example, the Inspector).
Technically trade-off aside, UnoCSS is also designed to be fully extensible and customizable, while Tailwind CSS is more opinionated. Building a custom design system (or design tokens) on top of Tailwind CSS can be hard, and you can't really move away from the Tailwind CSS's conventions. With UnoCSS, you could build pretty much anything you want with full control. For example, we implemented the whole Tailwind CSS compatible utilities within a single preset, and there are a lot of awesome community presets implements with other interesting philosophies.
Thanks to the flexibility UnoCSS provides, we are able to experiments with a lot innovative features on top of it, for example:
The downside of UnoCSS over Tailwind CSS is that it does not support Tailwind's plugin system or configurations, meaning it might make the migration harder from a heavily customized Tailwind CSS project. This is intended decision to make UnoCSS high-performant and extensible, and we believe the trade-off is worth it.