Style directives

text/0000-style-directives.md

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+- Start Date: 2020-12-09
+- RFC PR: (leave this empty)
+- Svelte Issue: (leave this empty)
+
+# Style Directive
+
+## Summary
+
+Add a `style:` directive:
+
+```svelte
+<div style:position="absolute" style:left="67px" />
+```
+
+## Motivation
+
+Currently, when inline style attributes need to be set based on multiple variables, the construction of a style attribute string is left to the developer.
+
+Constructing these attribute strings can become somewhat clumsy.
+
+For example:
+
+```svelte
+<div
+  style="
+    position: {position};
+    {position === 'absolute' ? 'top: 20px;' : ''}
+    {pointerEvents === false ? 'pointer-events:none;' : ''}
+  "
+/>
+```
+
+It would be useful — and often less error-prone — to be able to set multiple inline styles without having to construct the attribute string:
+
+```svelte
+<div
+  style:position="absolute"
+  style:top={position === "absolute" ? "20px" : null}
+  style:pointer-events={pointerEvents ? null : "none"}
+/>
+```
+
+## Detailed design
+
+Style directives would take the form `style:property="value"` where `property` is a CSS property and `value` is a CSS property value.
+
+### Shorthand
+
+A shorthand form could also exist when a variable name and CSS property name match:
+
+```svelte
+<script>
+  const left = "234px";
+</script>
+<div style:left />
+```
+
+### Conflicting Styles
+
+One question surrounding this approach how to combine style directives with style attribures that have conflicting properties.
+
+Consider the example:
+
+```svelte
+<div style="position: absolute; left: 40px;" style:left="50px" />
+```
+
+What would be the value of this element's `left` property?
+
+It seems there are three options available:
+
+1. Conflicting CSS properties in style directives are not allowed, and a compiler error is thrown.
+2. The style attribute takes precedence.
+3. The style directive takes precedence.
+
+The third option seems like the most reasonable, and most closely matches the behavior of the `class:` directive.
+
+For example, class directives take precedence over class attribute strings:
+
+```svelte
+<div class="foo bar" class:bar={false}>
+  My class is "foo" and not "foo bar"
+</div>
+```
+
+### CSS Variables
+
+These directives could also contain CSS variables:
+
+```svelte
+<div style:--border-color="saddleBrown" />
+```
+
+## How we teach this
+
+This is pretty similar to the `class:` directive, except that a CSS value is used instead of a boolean.
+
+## Drawbacks
+
+As with any feature, this would add some complexity to the codebase, and add a bit of learning curve, although the impact seems minimal.
+
+We may want to discourage the construction of complex inline styles based on variables, and encourage the use of CSS variables instead. 
+Of course, dynamic CSS variables would also need to be passed into a style attribute. 
+
+## Alternatives
+
+React uses a style object:
+
+```jsx
+<div style={{ marginLeft: "30px" }} />
+```
+
+We could instead mimic React and allow this sort of object to be passed to an element's `style` attribute.
+
+## Unresolved questions
+
+### Camel Case Property Names?
+
+We could also allow for the use of camel case property names. 
+
+This would make using variable names that match CSS properties a bit easier:
+
+```svelte
+<script>
+  const marginLeft = "20px";
+</script>
+<div style:marginLeft />
+```
+