fix: handle autofocus in `set_attributes`

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -431,7 +431,16 @@ function set_attributes(
 			// avoid using the setter
 			set_attribute(element, key, value);
 		} else if (key === 'autofocus') {
+			if (renderer == null) {
 				autofocus(/** @type {HTMLElement} */ (element), Boolean(value));
+			} else {
+				// In custom renderer mode, just set autofocus as a regular attribute
+				if (value) {
+					set_attribute_op(element, key, value);
+				} else {
+					remove_attribute(element, key);
+				}
+			}
 		} else if (!is_custom_element && (key === '__value' || (key === 'value' && value != null))) {
 			// @ts-ignore We're not running this for custom elements because __value is actually
 			// how Lit stores the current value on the element, and messing with that would break things.

packages/svelte/tests/custom-renderers/samples/svelte-element-autofocus/_config.js

@@ -0,0 +1,18 @@
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		// If we got here, the component mounted without crashing on document.body access.
+		// Verify autofocus is set as a regular attribute.
+		const input = target.children.find(
+			(/** @type {any} */ n) => n.type === 'element' && n.name === 'input'
+		);
+		assert.ok(input, 'input element should exist');
+		assert.equal(
+			input.attributes['autofocus'],
+			'true',
+			'autofocus should be set as a regular attribute'
+		);
+		assert.equal(input.attributes['value'], 'test', 'value should be set as a regular attribute');
+	}
+});

packages/svelte/tests/custom-renderers/samples/svelte-element-autofocus/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	let input_tag = 'input';
+</script>
+
+<svelte:element this={input_tag} autofocus value="test" />