diff --git a/packages/svelte/src/internal/client/constants.js b/packages/svelte/src/internal/client/constants.js
index df96f4899b..032b6c3e2b 100644
--- a/packages/svelte/src/internal/client/constants.js
+++ b/packages/svelte/src/internal/client/constants.js
@@ -69,6 +69,7 @@ export const STALE_REACTION = new (class StaleReactionError extends Error {
 	message = 'The reaction that called `getAbortSignal()` was re-run or destroyed';
 })();
 
+// TODO: DOM access
 export const IS_XHTML =
 	// We gotta write it like this because after downleveling the pure comment may end up in the wrong location
 	!!globalThis.document?.contentType &&
@@ -77,3 +78,10 @@ export const ELEMENT_NODE = 1;
 export const TEXT_NODE = 3;
 export const COMMENT_NODE = 8;
 export const DOCUMENT_FRAGMENT_NODE = 11;
+
+export const CUSTOM_RENDERER_NODE_TYPE_MAP = /** @type {const} */ ({
+	fragment: DOCUMENT_FRAGMENT_NODE,
+	element: ELEMENT_NODE,
+	text: TEXT_NODE,
+	comment: COMMENT_NODE
+});
diff --git a/packages/svelte/src/internal/client/dom/operations.js b/packages/svelte/src/internal/client/dom/operations.js
index 4036aa2d61..52c68291ca 100644
--- a/packages/svelte/src/internal/client/dom/operations.js
+++ b/packages/svelte/src/internal/client/dom/operations.js
@@ -5,9 +5,15 @@ import { init_array_prototype_warnings } from '../dev/equality.js';
 import { get_descriptor, is_extensible } from '../../shared/utils.js';
 import { active_effect } from '../runtime.js';
 import { async_mode_flag } from '../../flags/index.js';
-import { TEXT_NODE, REACTION_RAN } from '#client/constants';
+import {
+	TEXT_NODE,
+	REACTION_RAN,
+	CUSTOM_RENDERER_NODE_TYPE_MAP,
+	COMMENT_NODE
+} from '#client/constants';
 import { eager_block_effects } from '../reactivity/batch.js';
 import { NAMESPACE_HTML } from '../../../constants.js';
+import { renderer } from '../custom-renderer/state.js';
 
 // export these for reference in the compiled code, making global name deduplication unnecessary
 /** @type {Window} */
@@ -78,6 +84,7 @@ export function init_operations() {
  * @returns {Text}
  */
 export function create_text(value = '') {
+	if (renderer) return /** @type {Text} */ (renderer.createTextNode(value));
 	return document.createTextNode(value);
 }
 
@@ -87,6 +94,7 @@ export function create_text(value = '') {
  */
 /*@__NO_SIDE_EFFECTS__*/
 export function get_first_child(node) {
+	if (renderer) return /** @type {TemplateNode | null} */ (renderer.getFirstChild(node));
 	return /** @type {TemplateNode | null} */ (first_child_getter.call(node));
 }
 
@@ -96,6 +104,7 @@ export function get_first_child(node) {
  */
 /*@__NO_SIDE_EFFECTS__*/
 export function get_next_sibling(node) {
+	if (renderer) return /** @type {TemplateNode | null} */ (renderer.getNextSibling(node));
 	return /** @type {TemplateNode | null} */ (next_sibling_getter.call(node));
 }
 
@@ -115,10 +124,10 @@ export function child(node, is_text) {
 
 	// Child can be null if we have an element with a single child, like `<p>{text}</p>`, where `text` is empty
 	if (child === null) {
-		child = hydrate_node.appendChild(create_text());
-	} else if (is_text && child.nodeType !== TEXT_NODE) {
+		child = /** @type {TemplateNode} */ (append_child(hydrate_node, create_text()));
+	} else if (is_text && node_type(child) !== TEXT_NODE) {
 		var text = create_text();
-		child?.before(text);
+		insert_before(child, text);
 		set_hydrate_node(text);
 		return text;
 	}
@@ -142,7 +151,8 @@ export function first_child(node, is_text = false) {
 		var first = get_first_child(node);
 
 		// TODO prevent user comments with the empty string when preserveComments is true
-		if (first instanceof Comment && first.data === '') return get_next_sibling(first);
+		// TODO RENDERER: can we find another way to not have an `isComment` on the renderer?
+		if (is_comment(first) && get_node_value(first) === '') return get_next_sibling(first);
 
 		return first;
 	}
@@ -150,10 +160,10 @@ export function first_child(node, is_text = false) {
 	if (is_text) {
 		// if an {expression} is empty during SSR, there might be no
 		// text node to hydrate — we must therefore create one
-		if (hydrate_node?.nodeType !== TEXT_NODE) {
+		if (node_type(hydrate_node) !== TEXT_NODE) {
 			var text = create_text();
 
-			hydrate_node?.before(text);
+			if (hydrate_node) insert_before(hydrate_node, text);
 			set_hydrate_node(text);
 			return text;
 		}
@@ -187,15 +197,15 @@ export function sibling(node, count = 1, is_text = false) {
 	if (is_text) {
 		// if a sibling {expression} is empty during SSR, there might be no
 		// text node to hydrate — we must therefore create one
-		if (next_sibling?.nodeType !== TEXT_NODE) {
+		if (node_type(next_sibling) !== TEXT_NODE) {
 			var text = create_text();
 			// If the next sibling is `null` and we're handling text then it's because
 			// the SSR content was empty for the text, so we need to generate a new text
 			// node and insert it after the last sibling
 			if (next_sibling === null) {
-				last_sibling?.after(text);
+				if (last_sibling) insert_after(last_sibling, text);
 			} else {
-				next_sibling.before(text);
+				insert_before(next_sibling, text);
 			}
 			set_hydrate_node(text);
 			return text;
@@ -214,6 +224,15 @@ export function sibling(node, count = 1, is_text = false) {
  * @returns {void}
  */
 export function clear_text_content(node) {
+	if (renderer) {
+		var child = renderer.getFirstChild(node);
+		while (child !== null) {
+			var next = renderer.getNextSibling(child);
+			renderer.remove(child);
+			child = next;
+		}
+		return;
+	}
 	node.textContent = '';
 }
 
@@ -239,6 +258,10 @@ export function should_defer_append() {
  * @returns {T extends keyof HTMLElementTagNameMap ? HTMLElementTagNameMap[T] : Element}
  */
 export function create_element(tag, namespace, is) {
+	if (renderer)
+		return /** @type {T extends keyof HTMLElementTagNameMap ? HTMLElementTagNameMap[T] : Element} */ (
+			renderer.createElement(tag)
+		);
 	let options = is ? { is } : undefined;
 	return /** @type {T extends keyof HTMLElementTagNameMap ? HTMLElementTagNameMap[T] : Element} */ (
 		document.createElementNS(namespace ?? NAMESPACE_HTML, tag, options)
@@ -246,6 +269,7 @@ export function create_element(tag, namespace, is) {
 }
 
 export function create_fragment() {
+	if (renderer) return /** @type {DocumentFragment} */ (renderer.createFragment());
 	return document.createDocumentFragment();
 }
 
@@ -254,6 +278,7 @@ export function create_fragment() {
  * @returns
  */
 export function create_comment(data = '') {
+	if (renderer) return /** @type {Comment} */ (renderer.createComment(data));
 	return document.createComment(data);
 }
 
@@ -264,6 +289,10 @@ export function create_comment(data = '') {
  * @returns
  */
 export function set_attribute(element, key, value = '') {
+	if (renderer) {
+		renderer.setAttribute(element, key, value);
+		return;
+	}
 	if (key.startsWith('xlink:')) {
 		element.setAttributeNS('http://www.w3.org/1999/xlink', key, value);
 		return;
@@ -277,13 +306,16 @@ export function set_attribute(element, key, value = '') {
  * @param {Text} text
  */
 export function merge_text_nodes(text) {
+	// if we have a renderer we will not hydrate so we can skip this
+	if (renderer) return;
+
 	if (/** @type {string} */ (text.nodeValue).length < 65536) {
 		return;
 	}
 
 	let next = text.nextSibling;
 
-	while (next !== null && next.nodeType === TEXT_NODE) {
+	while (next !== null && node_type(next) === TEXT_NODE) {
 		next.remove();
 
 		/** @type {string} */ (text.nodeValue) += /** @type {string} */ (next.nodeValue);
@@ -291,3 +323,438 @@ export function merge_text_nodes(text) {
 		next = text.nextSibling;
 	}
 }
+
+/**
+ * @param {TemplateNode | null} node
+ * @returns {node is Comment}
+ */
+function is_comment(node) {
+	if (renderer) return !!node && node_type(node) === COMMENT_NODE;
+	return node instanceof Comment;
+}
+
+/**
+ * @param {Node | null | undefined} node
+ */
+export function node_type(node) {
+	if (node == null) return undefined;
+	if (renderer) {
+		const type = renderer.nodeType(node);
+		return CUSTOM_RENDERER_NODE_TYPE_MAP[type];
+	}
+	return node?.nodeType;
+}
+
+/**
+ * @param {Node | null | undefined} node
+ */
+export function node_name(node) {
+	if (node == null) return undefined;
+	if (renderer) {
+		return '';
+	}
+	return node?.nodeName;
+}
+
+/**
+ * @param {Node} node
+ * @returns {TemplateNode | null}
+ */
+export function get_last_child(node) {
+	if (renderer) return /** @type {TemplateNode | null} */ (renderer.getLastChild(node));
+	return /** @type {TemplateNode | null} */ (node.lastChild);
+}
+
+/**
+ * @param {Node} node
+ * @returns {TemplateNode | null}
+ */
+export function get_parent_node(node) {
+	if (renderer) return /** @type {TemplateNode | null} */ (renderer.getParent(node));
+	return /** @type {TemplateNode | null} */ (node.parentNode);
+}
+
+/**
+ * @param {Node} parent
+ * @param {Node} child
+ * @returns {Node}
+ */
+export function append_child(parent, child) {
+	if (renderer) {
+		renderer.insert(parent, child, null);
+		return child;
+	}
+	return parent.appendChild(child);
+}
+
+/**
+ * Insert `new_node` before `ref_node` (equivalent to `ref_node.before(new_node)`)
+ * @param {ChildNode} ref_node
+ * @param {Node} new_node
+ */
+export function insert_before(ref_node, new_node) {
+	if (renderer) {
+		var parent = renderer.getParent(ref_node);
+		renderer.insert(parent, new_node, ref_node);
+		return;
+	}
+	ref_node.before(new_node);
+}
+
+/**
+ * Insert `new_node` after `ref_node` (equivalent to `ref_node.after(new_node)`)
+ * @param {ChildNode} ref_node
+ * @param {Node} new_node
+ */
+export function insert_after(ref_node, new_node) {
+	if (renderer) {
+		var parent = renderer.getParent(ref_node);
+		var next = renderer.getNextSibling(ref_node);
+		renderer.insert(parent, new_node, next);
+		return;
+	}
+	ref_node.after(new_node);
+}
+
+/**
+ * @param {ChildNode} node
+ */
+export function remove_node(node) {
+	if (renderer) {
+		renderer.remove(node);
+		return;
+	}
+	node.remove();
+}
+
+/**
+ * @param {Node} parent
+ * @param {ChildNode} child
+ * @returns {ChildNode}
+ */
+export function remove_child(parent, child) {
+	if (renderer) {
+		renderer.remove(child);
+		return child;
+	}
+	return /** @type {ChildNode} */ (parent.removeChild(child));
+}
+
+/**
+ * @param {ChildNode} old_node
+ * @param {Node} new_node
+ */
+export function replace_with(old_node, new_node) {
+	if (renderer) {
+		var parent = renderer.getParent(old_node);
+		renderer.insert(parent, new_node, old_node);
+		renderer.remove(old_node);
+		return;
+	}
+	old_node.replaceWith(new_node);
+}
+
+/**
+ * @param {Node} node
+ * @param {string} value
+ */
+export function set_text_content(node, value) {
+	if (renderer) {
+		renderer.setText(node, value);
+		return;
+	}
+	node.textContent = value;
+}
+
+/**
+ * @param {Node} node
+ * @param {string} value
+ */
+export function set_node_value(node, value) {
+	if (renderer) {
+		renderer.setText(node, value);
+		return;
+	}
+	node.nodeValue = value;
+}
+
+// --- Helpers for style attribute string manipulation (custom renderer) ---
+
+/**
+ * @param {string} style_string
+ * @param {string} property
+ * @param {string} value
+ * @param {string} [priority]
+ * @returns {string}
+ */
+function set_style_property_in_string(style_string, property, value, priority) {
+	var declaration = property + ': ' + value + (priority ? ' !' + priority : '');
+	var parts = style_string.split(';');
+	var found = false;
+
+	for (var i = 0; i < parts.length; i++) {
+		var colon_index = parts[i].indexOf(':');
+		if (colon_index !== -1 && parts[i].substring(0, colon_index).trim() === property) {
+			parts[i] = ' ' + declaration;
+			found = true;
+			break;
+		}
+	}
+
+	if (!found) {
+		parts.push(' ' + declaration);
+	}
+
+	return parts
+		.map((p) => p.trim())
+		.filter(Boolean)
+		.join('; ');
+}
+
+/**
+ * @param {string} style_string
+ * @param {string} property
+ * @returns {string}
+ */
+function remove_style_property_in_string(style_string, property) {
+	return style_string
+		.split(';')
+		.filter((part) => {
+			var colon_index = part.indexOf(':');
+			if (colon_index === -1) return false;
+			return part.substring(0, colon_index).trim() !== property;
+		})
+		.map((p) => p.trim())
+		.filter(Boolean)
+		.join('; ');
+}
+
+/**
+ * @param {Node} node
+ * @returns {string | null}
+ */
+export function get_node_value(node) {
+	if (renderer) return renderer.getNodeValue(node);
+	return node.nodeValue;
+}
+
+/**
+ * @param {Element} element
+ * @param {string} name
+ * @returns {string | null}
+ */
+export function get_attribute(element, name) {
+	if (renderer) return renderer.getAttribute(element, name);
+	return element.getAttribute(name);
+}
+
+/**
+ * @param {Element} element
+ * @param {string} name
+ */
+export function remove_attribute(element, name) {
+	if (renderer) {
+		renderer.removeAttribute(element, name);
+		return;
+	}
+	element.removeAttribute(name);
+}
+
+/**
+ * @param {Element} element
+ * @param {string} name
+ * @returns {boolean}
+ */
+export function has_attribute(element, name) {
+	// TODO RENDERER: could be worth removing and just using get?
+
+	if (renderer) return renderer.hasAttribute(element, name);
+	return element.hasAttribute(name);
+}
+
+/**
+ * @param {Element} element
+ * @param {string} value
+ */
+export function set_inner_html(element, value) {
+	if (renderer) {
+		throw new Error('setInnerHTML is not supported with custom renderers');
+	}
+	element.innerHTML = value;
+}
+
+/**
+ * @param {Node} node
+ * @param {boolean} deep
+ * @returns {Node}
+ */
+export function clone_node(node, deep) {
+	if (renderer) return renderer.cloneNode(node, deep);
+	return node.cloneNode(deep);
+}
+
+/**
+ * @param {Node} node
+ * @param {boolean} deep
+ * @returns {Node}
+ */
+export function import_node(node, deep) {
+	if (renderer) return renderer.cloneNode(node, deep);
+	return document.importNode(node, deep);
+}
+
+/**
+ * @param {EventTarget} target
+ * @param {string} type
+ * @param {EventListenerOrEventListenerObject} handler
+ * @param {boolean | AddEventListenerOptions} [options]
+ */
+export function add_event_listener(target, type, handler, options) {
+	if (renderer) {
+		renderer.addEventListener(target, type, handler, options);
+		return;
+	}
+	target.addEventListener(type, handler, options);
+}
+
+/**
+ * @param {EventTarget} target
+ * @param {string} type
+ * @param {EventListenerOrEventListenerObject} handler
+ * @param {boolean | EventListenerOptions} [options]
+ */
+export function remove_event_listener(target, type, handler, options) {
+	if (renderer) {
+		renderer.removeEventListener(target, type, handler, options);
+		return;
+	}
+	target.removeEventListener(type, handler, options);
+}
+
+/**
+ * @param {EventTarget} target
+ * @param {Event} event
+ * @returns {boolean}
+ */
+export function dispatch_event(target, event) {
+	if (renderer) {
+		// is only used in SSR which is not a thing for custom renderers
+		throw new Error('dispatchEvent is not supported with custom renderers');
+	}
+	return target.dispatchEvent(event);
+}
+
+/**
+ * @param {HTMLElement} element
+ * @param {string} property
+ * @param {string} value
+ * @param {string} [priority]
+ */
+export function style_set_property(element, property, value, priority) {
+	if (renderer) {
+		var style = renderer.getAttribute(element, 'style') || '';
+		var updated = set_style_property_in_string(style, property, value, priority);
+		renderer.setAttribute(element, 'style', updated);
+		return;
+	}
+	element.style.setProperty(property, value, priority);
+}
+
+/**
+ * @param {HTMLElement} element
+ * @param {string} property
+ */
+export function style_remove_property(element, property) {
+	if (renderer) {
+		var style = renderer.getAttribute(element, 'style') || '';
+		var updated = remove_style_property_in_string(style, property);
+		renderer.setAttribute(element, 'style', updated);
+		return;
+	}
+	element.style.removeProperty(property);
+}
+
+/**
+ * @param {HTMLElement} element
+ * @param {string} value
+ */
+export function set_css_text(element, value) {
+	if (renderer) {
+		renderer.setAttribute(element, 'style', value);
+		return;
+	}
+	element.style.cssText = value;
+}
+
+/**
+ * @param {Element} element
+ * @param {string} name
+ * @param {boolean} force
+ */
+export function class_list_toggle(element, name, force) {
+	if (renderer) {
+		const classes = element.getAttribute('class')?.split(/\s+/) ?? [];
+		const has_class = classes.includes(name);
+		if (force === has_class) {
+			return;
+		}
+		if (force) {
+			classes.push(name);
+		} else {
+			const index = classes.indexOf(name);
+			if (index !== -1) {
+				classes.splice(index, 1);
+			}
+		}
+		element.setAttribute('class', classes.join(' '));
+		return;
+	}
+	element.classList.toggle(name, force);
+}
+
+/**
+ * @param {Element} element
+ * @param {string} selector
+ */
+export function query_selector(element, selector) {
+	if (renderer) {
+		// TODO RENDERER: just used with css inject which doesn't make sense with custom renderers
+		throw new Error('querySelector is not supported with custom renderers');
+	}
+	return element.querySelector(selector);
+}
+
+/**
+ * @param {Element | DocumentFragment} element
+ * @param {string} selector
+ */
+export function query_selector_all(element, selector) {
+	if (renderer) {
+		// TODO RENDERER: just used with css inject which doesn't make sense with custom renderers
+		throw new Error('querySelectorAll is not supported with custom renderers');
+	}
+	return element.querySelectorAll(selector);
+}
+
+/**
+ * @param {Node} node
+ * @returns {Node}
+ */
+export function get_root_node(node) {
+	if (renderer) {
+		// TODO RENDERER: just used with css inject which doesn't make sense with custom renderers
+		throw new Error('getRootNode is not supported with custom renderers');
+	}
+	return node.getRootNode();
+}
+
+/**
+ * @param {HTMLElement} element
+ */
+export function focus(element) {
+	if (renderer) {
+		// doesn't make sense with custom renderers
+		throw new Error('focus is not supported with custom renderers');
+	}
+	element.focus();
+}