first version of CSSOM RFC

text/0000-cssom.md

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+- Start Date: 2020-04-20
+- RFC PR:
+- React Issue:
+
+# Summary
+
+I know it's a long-lasting and controversial topic and probably not the highest priority for the team right now, but let me try to create the first **simplified** vision for a CSS implementation we all could agree on.
+
+CSS-in-JS like React is on a dual mission, supporting both complex apps and small(ish) sites. This has created a big divide in the community because of the very different needs that these types of projects have. Like React, CSS-in-JS was born out of the need to reduce the complexity of maintaining large applications.
+
+While there are many options for authoring CSS, it is super tiresome to convince a team to use a particular approach. It creates a big divide and lots of frustration. While there is a huge value in not having a built-in solution and giving the space to 3rd party libs to thrive and experiment, the friction and frustration for not having a good built-in approach that covers 90% of use cases is real.
+
+## Motivation
+
+### Goals
+
+- High-performance rendering of static styles via CSSOM.
+- CSS Rules can be reused across components.
+- Components have no additional overhead for styling.
+- Lists - inline styles have no way to be reused across list elements, where typically styles are repeated.
+- Support the full CSS spec. Inline styles are limited to [CSSStyleDeclaration](https://developer.mozilla.org/en-US/docs/Web/API/CSSStyleDeclaration) and we need to support the full spec on the web.
+- Adhere to React's component model and the upcoming Suspense architecture.
+
+### Non-goals
+
+- Enforcing CSS-in-JS over vanilla CSS or css-modules (you can still use className)
+- Implementing every possible idea CSS-in-JS libs came up with
+
+# Detailed design
+
+### Tagged templates
+
+- familiar syntax
+- syntax highlighting (already supported)
+- linting
+- ability to return a data structure that React can identify, not just a string
+- interpolations
+
+```js
+import { css } from "react-cssom";
+const buttonStyle = css`
+  color: red;
+`; // {css: '.css-0 { color: red; }'}
+
+render(<button style={buttonStyle}>x</button>);
+```
+
+### Using style prop for both inline and CSSOM styles
+
+Since we can return from the tag function any data structure we want, we can identify styles created using our `css` function and we have no problem accepting both types of styles.
+
+Using the `style` prop allows us to be flexible in the future about how React wants to optimize and treat this interface.
+
+That being said using a new prop like `css` is also an option:
+
+```js
+render(<button css={buttonStyle}>x</button>);
+```
+
+### Babel plugin
+
+Babel plugin can be optional and can enable advanced features like:
+
+- vendor prefixing
+- optimizations
+- other preprocessing options e.g. using PostCSS
+
+After babel plugin the call into `css` tag function can be removed and the result of the above example can be compiled to:
+
+```js
+const buttonStyle = { css: ".css-0 { color: red; }" };
+render(<button style={buttonStyle}>x</button>);
+```
+
+# Drawbacks
+
+- we are adding a new area of responsibility to React, which will require more work to maintain it, but hopefully can be parallelized
+- it inevitably will raise the discussions about how React is enforcing CSS-in-JS, so we will have to prepare a good answer
+- there are several of unresolved features listed below, which will have to be addressed over time
+
+# Alternatives
+
+This is a very simple initial version, so it's not as feature-rich as some other CSS-in-JS solutions, but it has a potential to cover more use cases later.
+
+It is not a replacement for react-native approach.
+
+It is not a replacement for any entirely different approaches like SwiftUI.
+
+# Adoption strategy
+
+It is not a breaking change.
+
+# How we teach this
+
+It is a primitive interface for passing CSS to React to integrate with component's lifecycle. It can be also a way for babel plugins and compile-time processing of CSS for React components.
+
+# Unresolved questions
+
+- Composition
+- Overrides
+- Dynamic or state-based styling
+- Theming
+- Alternative object-based syntax
+- Compatibility with react-native