robustify for multiple blocks:

- revert to previous value on unmount, if it was the last one changing the value
- special handling for classes: merge them

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

@@ -1,8 +1,7 @@
-/** @import { ExpressionStatement } from 'estree' */
+/** @import { ExpressionStatement, Property } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types' */
-import { is_dom_property, normalize_attribute } from '../../../../../utils.js';
-import { is_ignored } from '../../../../state.js';
+import { normalize_attribute } from '../../../../../utils.js';
 import { is_event_attribute } from '../../../../utils/ast.js';
 import * as b from '../../../../utils/builders.js';
 import { build_attribute_value } from './shared/element.js';
@@ -13,7 +12,8 @@ import { visit_event_attribute } from './shared/events.js';
  * @param {ComponentContext} context
  */
 export function SvelteHTML(element, context) {
-	const node_id = b.id('$.document.documentElement');
+	/** @type {Property[]} */
+	const attributes = [];
 
 	for (const attribute of element.attributes) {
 		if (attribute.type === 'Attribute') {
@@ -21,32 +21,9 @@ export function SvelteHTML(element, context) {
 				visit_event_attribute(attribute, context);
 			} else {
 				const name = normalize_attribute(attribute.name);
-				const { value, has_state } = build_attribute_value(attribute.value, context);
+				const { value } = build_attribute_value(attribute.value, context);
 
-				/** @type {ExpressionStatement} */
-				let update;
-
-				if (name === 'class') {
-					update = b.stmt(b.call('$.set_class', node_id, value));
-				} else if (is_dom_property(name)) {
-					update = b.stmt(b.assignment('=', b.member(node_id, name), value));
-				} else {
-					update = b.stmt(
-						b.call(
-							'$.set_attribute',
-							node_id,
-							b.literal(name),
-							value,
-							is_ignored(element, 'hydration_attribute_changed') && b.true
-						)
-					);
-				}
-
-				if (has_state) {
-					context.state.update.push(update);
-				} else {
-					context.state.init.push(update);
-				}
+				attributes.push(b.init(name, value));
 
 				if (context.state.options.dev) {
 					context.state.init.push(
@@ -56,4 +33,8 @@ export function SvelteHTML(element, context) {
 			}
 		}
 	}
+
+	if (attributes.length > 0) {
+		context.state.init.push(b.stmt(b.call('$.svelte_html', b.arrow([], b.object(attributes)))));
+	}
 }

packages/svelte/src/internal/client/dom/blocks/svelte-html.js

@@ -0,0 +1,54 @@
+import { render_effect, teardown } from '../../reactivity/effects.js';
+import { set_attribute } from '../elements/attributes.js';
+import { set_class } from '../elements/class.js';
+import { hydrating } from '../hydration.js';
+
+/**
+ * @param {() => Record<string, any>} get_attributes
+ * @returns {void}
+ */
+export function svelte_html(get_attributes) {
+	const node = document.documentElement;
+	const own = {};
+
+	/** @type {Record<string, Array<[any, any]>>} to check who set the last value of each attribute */
+	// @ts-expect-error
+	const current_setters = (node.__attributes_setters ??= {});
+
+	/** @type {Record<string, any>} */
+	let attributes;
+
+	render_effect(() => {
+		attributes = get_attributes();
+
+		for (const name in attributes) {
+			let value = attributes[name];
+			current_setters[name] = (current_setters[name] ?? []).filter(([owner]) => owner !== own);
+			current_setters[name].unshift([own, value]);
+
+			// Do nothing on initial render during hydration: If there are attribute duplicates, the last value
+			// wins, which could result in needless hydration repairs from earlier values.
+			if (hydrating) continue;
+
+			if (name === 'class') {
+				set_class(node, current_setters[name].map(([_, value]) => value).join(' '));
+			} else {
+				set_attribute(node, name, value);
+			}
+		}
+	});
+
+	teardown(() => {
+		for (const name in attributes) {
+			const old = current_setters[name];
+			current_setters[name] = old.filter(([owner]) => owner !== own);
+			const current = current_setters[name];
+
+			if (name === 'class') {
+				set_class(node, current.map(([_, value]) => value).join(' '));
+			} else if (old[0][0] === own) {
+				set_attribute(node, name, current[0]?.[1]);
+			}
+		}
+	});
+}

packages/svelte/src/internal/client/index.js

@@ -24,6 +24,7 @@ export { snippet, wrap_snippet } from './dom/blocks/snippet.js';
 export { component } from './dom/blocks/svelte-component.js';
 export { element } from './dom/blocks/svelte-element.js';
 export { head } from './dom/blocks/svelte-head.js';
+export { svelte_html } from './dom/blocks/svelte-html.js';
 export { append_styles } from './dom/css.js';
 export { action } from './dom/elements/actions.js';
 export {
@@ -149,7 +150,12 @@ export {
 	setContext,
 	hasContext
 } from './runtime.js';
-export { validate_binding, validate_each_keys, validate_prop_bindings } from './validate.js';
+export {
+	validate_binding,
+	validate_each_keys,
+	validate_prop_bindings,
+	validate_svelte_html_attribute
+} from './validate.js';
 export { raf } from './timing.js';
 export { proxy } from './proxy.js';
 export { create_custom_element } from './dom/elements/custom-element.js';

packages/svelte/src/internal/server/blocks/svelte-html.js

@@ -9,9 +9,17 @@ import { svelte_html_duplicate_attribute } from '../../shared/warnings.js';
  */
 export function svelte_html(payload, attributes) {
 	for (const name in attributes) {
+		let value = attributes[name];
+
 		if (payload.htmlAttributes.has(name)) {
+			if (name === 'class') {
+				// Don't bother deduplicating class names, the browser handles it just fine
+				value = `${payload.htmlAttributes.get(name)} ${value}`;
+			} else {
 				svelte_html_duplicate_attribute(name);
 			}
-		payload.htmlAttributes.set(name, escape_html(attributes[name], true));
+		}
+
+		payload.htmlAttributes.set(name, escape_html(value, true));
 	}
 }