add StyleValue into doc

Author: adiguba

documentation/docs/03-template-syntax/17-style.md

@@ -1,7 +1,75 @@
 ---
-title: style:
+title: style and style:
 ---
 
+There are two ways to set styles on elements: the `style` attribute, and the `style:` directive.
+
+## Attributes
+
+Primitive values are treated like any other attribute:
+
+```svelte
+<div style={big ? 'font-size:2em' : 'font-size:1.2em'}>...</div>
+```
+
+### Objects and arrays
+
+Since Svelte 5.XX, `style` can be an object or array, and is converted to a string according to the following rules :
+
+If the value is an 
+
+If the value is an object, the key/value are converted to CSS properties if the value is not-null and not-empty.
+
+```svelte
+<!-- equivalent to <div style="color:red;display:inline"> -->
+<div style={{ color: 'red', display: 'inline', background: null }}>...</div>
+```
+
+> [!NOTE]
+> The CSS properties are case-insensitive and use `kebab-case`, which requires quoting key's name in JavaScript.
+> In order to avoid this, object keys will be 'converted' according to the following rules :
+> * Uppercase keys like `COLOR` will be converted to the lowercase format `color`.
+> * `camelCase` keys like `fontSize` will be converted to the kebab-case format `font-size`.
+> * `snake_case` keys like `border_color` will be converted to the kebab-case format `border-color`.
+> Note that this will not apply to key that starts with a double hyphens, because CSS variable don't have naming rules and are case-sensitive (`--myvar` is different from `--myVar`).
+> But we can use a double underscores to enable the same rules. Ex: `__myVar` or `__my_var` will be converted to `--my-var`.
+
+If the value is an array, the truthy values are combined, string are passed without change, and array/objects are flatten :
+
+```svelte
+<!-- equivalent to <div style="color:red;display:inline;--my-var:0;font-size:2em;background: black"> -->
+<div style={['color:red', {display:'inline'}, [{__my_var: 0, fontSize: '2em'}, 'background: black']]}>...</div>
+```
+
+This is useful for combining local styles with props, for example:
+
+```svelte
+<!--- file: Button.svelte --->
+<script>
+	let props = $props();
+</script>
+
+<button {...props} style={[props.style, {color:'red', background:'black'}]}>
+	{@render props.children?.()}
+</button>
+```
+
+
+Svelte also exposes the `StyleValue` type, which is the type of value that the `style` attribute on elements accept. This is useful if you want to use a type-safe class name in component props:
+
+```svelte
+<script lang="ts">
+	import type { StyleValue } from 'svelte/elements';
+
+	const props: { style: StyleValue } = $props();
+</script>
+
+<div style={[props.style, {color: 'red'}]}>...</div>
+```
+
+
+## The `style:` directive
+
 The `style:` directive provides a shorthand for setting multiple styles on an element.
 
 ```svelte