diff --git a/.changeset/full-doors-stand.md b/.changeset/full-doors-stand.md
new file mode 100644
index 0000000000..15a4897b36
--- /dev/null
+++ b/.changeset/full-doors-stand.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: always lowercase HTML elements, for XHTML compliance
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-template/template.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-template/template.js
index 8f7f8a1f43..f546ce4962 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-template/template.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-template/template.js
@@ -30,13 +30,15 @@ export class Template {
 	/**
 	 * @param {string} name
 	 * @param {number} start
+	 * @param {boolean} is_html
 	 */
-	push_element(name, start) {
+	push_element(name, start, is_html) {
 		this.#element = {
 			type: 'element',
 			name,
 			attributes: {},
 			children: [],
+			is_html,
 			start
 		};
 
@@ -100,7 +102,7 @@ function stringify(item) {
 	for (const key in item.attributes) {
 		const value = item.attributes[key];
 
-		str += ` ${key}`;
+		str += ` ${item.is_html ? key.toLowerCase() : key}`;
 		if (value !== undefined) str += `="${escape_html(value, true)}"`;
 	}
 
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-template/types.d.ts b/packages/svelte/src/compiler/phases/3-transform/client/transform-template/types.d.ts
index 1555e10ef0..2ccb4dc5c7 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-template/types.d.ts
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-template/types.d.ts
@@ -5,6 +5,7 @@ export interface Element {
 	name: string;
 	attributes: Record<string, string | undefined>;
 	children: Node[];
+	is_html: boolean;
 	/** used for populating __svelte_meta */
 	start: number;
 }
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
index 7469dcda5b..0579d80b74 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
@@ -38,9 +38,11 @@ import { TEMPLATE_FRAGMENT } from '../../../../../constants.js';
  * @param {ComponentContext} context
  */
 export function RegularElement(node, context) {
-	context.state.template.push_element(node.name, node.start);
+	const is_html = context.state.metadata.namespace === 'html' && node.name !== 'svg';
+	const name = is_html ? node.name.toLowerCase() : node.name;
+	context.state.template.push_element(name, node.start, is_html);
 
-	if (node.name === 'noscript') {
+	if (name === 'noscript') {
 		context.state.template.pop_element();
 		return;
 	}
@@ -53,9 +55,9 @@ export function RegularElement(node, context) {
 	// Therefore we need to use importNode instead, which doesn't have this caveat.
 	// Additionally, Webkit browsers need importNode for video elements for autoplay
 	// to work correctly.
-	context.state.template.needs_import_node ||= node.name === 'video' || is_custom_element;
+	context.state.template.needs_import_node ||= name === 'video' || is_custom_element;
 
-	context.state.template.contains_script_tag ||= node.name === 'script';
+	context.state.template.contains_script_tag ||= name === 'script';
 
 	/** @type {Array<AST.Attribute | AST.SpreadAttribute>} */
 	const attributes = [];
@@ -161,7 +163,7 @@ export function RegularElement(node, context) {
 		}
 	}
 
-	if (node.name === 'input') {
+	if (name === 'input') {
 		const has_value_attribute = attributes.some(
 			(attribute) =>
 				attribute.type === 'Attribute' &&
@@ -190,7 +192,7 @@ export function RegularElement(node, context) {
 		}
 	}
 
-	if (node.name === 'textarea') {
+	if (name === 'textarea') {
 		const attribute = lookup.get('value') ?? lookup.get('checked');
 		const needs_content_reset = attribute && !is_text_attribute(attribute);
 
@@ -206,10 +208,7 @@ export function RegularElement(node, context) {
 
 	/** If true, needs `__value` for inputs */
 	const needs_special_value_handling =
-		node.name === 'option' ||
-		node.name === 'select' ||
-		bindings.has('group') ||
-		bindings.has('checked');
+		name === 'option' || name === 'select' || bindings.has('group') || bindings.has('checked');
 
 	if (has_spread) {
 		build_attribute_effect(
@@ -258,7 +257,6 @@ export function RegularElement(node, context) {
 				let { value } = build_attribute_value(attribute.value, context);
 				context.state.init.push(b.stmt(b.call('$.autofocus', node_id, value)));
 			} else if (name === 'class') {
-				const is_html = context.state.metadata.namespace === 'html' && node.name !== 'svg';
 				build_set_class(node, node_id, attribute, class_directives, context, is_html);
 			} else if (name === 'style') {
 				build_set_style(node_id, attribute, style_directives, context);
@@ -279,7 +277,7 @@ export function RegularElement(node, context) {
 	}
 
 	if (
-		is_load_error_element(node.name) &&
+		is_load_error_element(name) &&
 		(has_spread || has_use || lookup.has('onload') || lookup.has('onerror'))
 	) {
 		context.state.after_update.push(b.stmt(b.call('$.replay_events', node_id)));
@@ -307,8 +305,7 @@ export function RegularElement(node, context) {
 		...context.state,
 		metadata,
 		scope: /** @type {Scope} */ (context.state.scopes.get(node.fragment)),
-		preserve_whitespace:
-			context.state.preserve_whitespace || node.name === 'pre' || node.name === 'textarea'
+		preserve_whitespace: context.state.preserve_whitespace || name === 'pre' || name === 'textarea'
 	};
 
 	const { hoisted, trimmed } = clean_nodes(
@@ -317,7 +314,7 @@ export function RegularElement(node, context) {
 		context.path,
 		state.metadata.namespace,
 		state,
-		node.name === 'script' || state.preserve_whitespace,
+		name === 'script' || state.preserve_whitespace,
 		state.options.preserveComments
 	);
 
@@ -363,7 +360,7 @@ export function RegularElement(node, context) {
 		context.state.template.push_comment();
 
 		// Create a separate template for the rich content
-		const template_name = context.state.scope.root.unique(`${node.name}_content`);
+		const template_name = context.state.scope.root.unique(`${name}_content`);
 		const fragment_id = b.id(context.state.scope.generate('fragment'));
 		const anchor_id = b.id(context.state.scope.generate('anchor'));
 
@@ -414,7 +411,7 @@ export function RegularElement(node, context) {
 
 		// The same applies if it's a `<template>` element, since we need to
 		// set the value of `hydrate_node` to `node.content`
-		if (node.name === 'template') {
+		if (name === 'template') {
 			needs_reset = true;
 			child_state.init.push(b.stmt(b.call('$.hydrate_template', arg)));
 			arg = b.member(arg, 'content');
@@ -451,7 +448,7 @@ export function RegularElement(node, context) {
 		context.state.after_update.push(...element_state.after_update);
 	}
 
-	if (node.name === 'selectedcontent') {
+	if (name === 'selectedcontent') {
 		context.state.init.push(
 			b.stmt(
 				b.call(
@@ -483,11 +480,11 @@ export function RegularElement(node, context) {
 			// this node is an `option` that didn't have a `value` attribute, but had
 			// a single-expression child, so we treat the value of that expression as
 			// the value of the option
-			build_element_special_value_attribute(node.name, node_id, synthetic_attribute, context, true);
+			build_element_special_value_attribute(name, node_id, synthetic_attribute, context, true);
 		} else {
 			for (const attribute of /** @type {AST.Attribute[]} */ (attributes)) {
 				if (attribute.name === 'value') {
-					build_element_special_value_attribute(node.name, node_id, attribute, context);
+					build_element_special_value_attribute(name, node_id, attribute, context);
 					break;
 				}
 			}
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js
index 1d6d3413bf..216bf08b5c 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js
@@ -488,10 +488,10 @@ export function build_component(node, component_name, loc, context) {
 	if (Object.keys(custom_css_props).length > 0) {
 		if (context.state.metadata.namespace === 'svg') {
 			// this boils down to <g><!></g>
-			context.state.template.push_element('g', node.start);
+			context.state.template.push_element('g', node.start, false);
 		} else {
 			// this boils down to <svelte-css-wrapper style='display: contents'><!></svelte-css-wrapper>
-			context.state.template.push_element('svelte-css-wrapper', node.start);
+			context.state.template.push_element('svelte-css-wrapper', node.start, false);
 			context.state.template.set_prop('style', 'display: contents');
 		}
 
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/RegularElement.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/RegularElement.js
index 1c6bb0a198..e653f545af 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/RegularElement.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/visitors/RegularElement.js
@@ -15,6 +15,7 @@ import { is_customizable_select_element } from '../../../nodes.js';
  * @param {ComponentContext} context
  */
 export function RegularElement(node, context) {
+	const name = context.state.namespace === 'html' ? node.name.toLowerCase() : node.name;
 	const namespace = determine_namespace_for_children(node, context.state.namespace);
 
 	/** @type {ComponentServerTransformState} */
@@ -27,7 +28,7 @@ export function RegularElement(node, context) {
 		template: []
 	};
 
-	const node_is_void = is_void(node.name);
+	const node_is_void = is_void(name);
 
 	const optimiser = new PromiseOptimiser();
 
@@ -35,28 +36,28 @@ export function RegularElement(node, context) {
 	// avoid calling build_element_attributes here to prevent evaluating/awaiting
 	// attribute expressions twice. We'll handle attributes in the special branch.
 	const is_select_special =
-		node.name === 'select' &&
+		name === 'select' &&
 		node.attributes.some(
 			(attribute) =>
 				((attribute.type === 'Attribute' || attribute.type === 'BindDirective') &&
 					attribute.name === 'value') ||
 				attribute.type === 'SpreadAttribute'
 		);
-	const is_option_special = node.name === 'option';
+	const is_option_special = name === 'option';
 	const is_special = is_select_special || is_option_special;
 
 	let body = /** @type {Expression | null} */ (null);
 	if (!is_special) {
 		// only open the tag in the non-special path
-		state.template.push(b.literal(`<${node.name}`));
+		state.template.push(b.literal(`<${name}`));
 		body = build_element_attributes(node, { ...context, state }, optimiser.transform);
 		state.template.push(b.literal(node_is_void ? '/>' : '>')); // add `/>` for XHTML compliance
 	}
 
-	if ((node.name === 'script' || node.name === 'style') && node.fragment.nodes.length === 1) {
+	if ((name === 'script' || name === 'style') && node.fragment.nodes.length === 1) {
 		state.template.push(
 			b.literal(/** @type {AST.Text} */ (node.fragment.nodes[0]).data),
-			b.literal(`</${node.name}>`)
+			b.literal(`</${name}>`)
 		);
 
 		context.state.template.push(
@@ -90,7 +91,7 @@ export function RegularElement(node, context) {
 				b.call(
 					'$.push_element',
 					b.id('$$renderer'),
-					b.literal(node.name),
+					b.literal(name),
 					b.literal(location.line),
 					b.literal(location.column)
 				)
@@ -142,7 +143,7 @@ export function RegularElement(node, context) {
 						b.call(
 							'$.push_element',
 							b.id('$$renderer'),
-							b.literal(node.name),
+							b.literal(name),
 							b.literal(location.line),
 							b.literal(location.column)
 						)
@@ -191,16 +192,13 @@ export function RegularElement(node, context) {
 	} else {
 		// For optgroup or select with rich content, add hydration marker at the start
 		process_children(trimmed, { ...context, state });
-		if (
-			(node.name === 'optgroup' || node.name === 'select') &&
-			is_customizable_select_element(node)
-		) {
+		if ((name === 'optgroup' || name === 'select') && is_customizable_select_element(node)) {
 			state.template.push(b.literal('<!>'));
 		}
 	}
 
 	if (!node_is_void) {
-		state.template.push(b.literal(`</${node.name}>`));
+		state.template.push(b.literal(`</${name}>`));
 	}
 
 	if (dev) {
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/element.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/element.js
index 5bc8fb15b5..ac02a6b918 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/element.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/element.js
@@ -235,7 +235,7 @@ export function build_element_attributes(node, context, transform) {
 
 				if (name !== 'class' || literal_value) {
 					context.state.template.push(
-						b.literal(` ${attribute.name}="${literal_value === true ? '' : String(literal_value)}"`)
+						b.literal(` ${name}="${literal_value === true ? '' : String(literal_value)}"`)
 					);
 				}
 
diff --git a/packages/svelte/tests/runtime-xhtml/samples/lowercase-tags/_config.js b/packages/svelte/tests/runtime-xhtml/samples/lowercase-tags/_config.js
new file mode 100644
index 0000000000..a92f216349
--- /dev/null
+++ b/packages/svelte/tests/runtime-xhtml/samples/lowercase-tags/_config.js
@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const [input] = target.querySelectorAll('input');
+
+		assert.equal(input.disabled, true);
+	}
+});
diff --git a/packages/svelte/tests/runtime-xhtml/samples/lowercase-tags/main.svelte b/packages/svelte/tests/runtime-xhtml/samples/lowercase-tags/main.svelte
new file mode 100644
index 0000000000..7c48b6bc4b
--- /dev/null
+++ b/packages/svelte/tests/runtime-xhtml/samples/lowercase-tags/main.svelte
@@ -0,0 +1 @@
+<iNPUT dIsAbLeD />