fix: don't lowercase attributes with custom renderers

packages/svelte/src/compiler/phases/3-transform/client/transform-template/template.js

@@ -137,7 +137,7 @@ function objectify(item) {
 		attributes.properties.push(
 			b.prop(
 				'init',
-				b.key(fix_attribute_casing(key)),
+				b.key(item.is_html ? fix_attribute_casing(key) : key),
 				value === undefined ? b.void0 : b.literal(value)
 			)
 		);

packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js

@@ -235,7 +235,7 @@ export function RegularElement(node, context) {
 				continue;
 			}
 
-			const name = get_attribute_name(node, attribute);
+			const name = get_attribute_name(node, attribute, !!context.state.analysis.custom_renderer);
 
 			if (
 				!is_custom_element &&

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/element.js

@@ -132,9 +132,10 @@ export function build_attribute_value(value, context, memoize = (value) => value
 /**
  * @param {AST.RegularElement | AST.SvelteElement} element
  * @param {AST.Attribute} attribute
+ * @param {boolean} [custom_renderer]
  */
-export function get_attribute_name(element, attribute) {
+export function get_attribute_name(element, attribute, custom_renderer) {
-	if (!element.metadata.svg && !element.metadata.mathml) {
+	if (!custom_renderer && !element.metadata.svg && !element.metadata.mathml) {
 		return normalize_attribute(attribute.name);
 	}
 

packages/svelte/src/internal/client/dom/operations.js

@@ -579,7 +579,7 @@ export function set_element_checked(element, checked) {
  */
 export function set_element_default_value(element, value) {
 	if (renderer) {
-		renderer.setAttribute(element, 'defaultvalue', value);
+		renderer.setAttribute(element, 'defaultValue', value);
 		return;
 	}
 	// @ts-expect-error
@@ -599,9 +599,9 @@ export function set_element_default_value(element, value) {
 export function set_element_default_checked(element, checked) {
 	if (renderer) {
 		if (checked) {
-			renderer.setAttribute(element, 'defaultchecked', '');
+			renderer.setAttribute(element, 'defaultChecked', '');
 		} else {
-			renderer.removeAttribute(element, 'defaultchecked');
+			renderer.removeAttribute(element, 'defaultChecked');
 		}
 		return;
 	}

packages/svelte/tests/custom-renderers/samples/attribute-casing/_config.js

@@ -0,0 +1,37 @@
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const elements = target.children.filter((/** @type {any} */ n) => n.type === 'element');
+
+		assert.equal(elements.length, 4);
+
+		// Static camelCase attribute should preserve casing
+		const div1 = elements[0];
+		assert.equal(div1.name, 'div');
+		assert.equal(div1.attributes['dataColor'], 'red');
+		// Should NOT have a lowercased version
+		assert.equal(div1.attributes['datacolor'], undefined);
+
+		// Dynamic camelCase attribute should preserve casing
+		const div2 = elements[1];
+		assert.equal(div2.name, 'div');
+		assert.equal(div2.attributes['viewBox'], '0 0 100 100');
+		// Should NOT have a lowercased version
+		assert.equal(div2.attributes['viewbox'], undefined);
+
+		// Static tabIndex should preserve casing
+		const span = elements[2];
+		assert.equal(span.name, 'span');
+		assert.equal(span.attributes['tabIndex'], '0');
+		// Should NOT have a lowercased version
+		assert.equal(span.attributes['tabindex'], undefined);
+
+		// Spread camelCase attributes should preserve casing
+		const p = elements[3];
+		assert.equal(p.name, 'p');
+		assert.equal(p.attributes['dataValue'], 'spread');
+		// Should NOT have a lowercased version
+		assert.equal(p.attributes['datavalue'], undefined);
+	}
+});

packages/svelte/tests/custom-renderers/samples/attribute-casing/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	let box = $state('0 0 100 100');
+	let spread = $derived({ dataValue: 'spread', onClick: () => {} });
+</script>
+
+<div dataColor="red">static camelCase</div>
+<div viewBox={box}>dynamic camelCase</div>
+<span tabIndex="0">static tabIndex</span>
+<p {...spread}>spread camelCase</p>

packages/svelte/tests/custom-renderers/samples/special-attributes/_config.js

@@ -27,16 +27,16 @@ export default test({
 		assert.equal(input_checked.attributes['type'], 'checkbox');
 		assert.equal(input_checked.attributes['checked'], '');
 
-		// Input 3: value="fixed" defaultvalue="default_val"
+		// Input 3: value="fixed" defaultValue="default_val"
 		const input_default_value = inputs[2];
 		assert.equal(input_default_value.attributes['value'], 'fixed');
-		assert.equal(input_default_value.attributes['defaultvalue'], 'default_val');
+		assert.equal(input_default_value.attributes['defaultValue'], 'default_val');
 
-		// Input 4: type="checkbox" checked="" defaultchecked=""
+		// Input 4: type="checkbox" checked="" defaultChecked=""
 		const input_default_checked = inputs[3];
 		assert.equal(input_default_checked.attributes['type'], 'checkbox');
 		assert.equal(input_default_checked.attributes['checked'], '');
-		assert.equal(input_default_checked.attributes['defaultchecked'], '');
+		assert.equal(input_default_checked.attributes['defaultChecked'], '');
 
 		// Input 5: spread attributes
 		const input_spread = inputs[4];
@@ -56,12 +56,12 @@ export default test({
 		// Input 2: checked should be removed
 		assert.equal(input_checked.attributes['checked'], undefined);
 
-		// Input 3: defaultvalue="new_default", value still "fixed"
+		// Input 3: defaultValue="new_default", value still "fixed"
 		assert.equal(input_default_value.attributes['value'], 'fixed');
-		assert.equal(input_default_value.attributes['defaultvalue'], 'new_default');
+		assert.equal(input_default_value.attributes['defaultValue'], 'new_default');
 
-		// Input 4: defaultchecked should be removed
+		// Input 4: defaultChecked should be removed
-		assert.equal(input_default_checked.attributes['defaultchecked'], undefined);
+		assert.equal(input_default_checked.attributes['defaultChecked'], undefined);
 
 		// Input 5: spread attributes should update value to "world"
 		assert.equal(input_spread.attributes['value'], 'world');