diff --git a/packages/svelte/src/internal/client/dom/operations.js b/packages/svelte/src/internal/client/dom/operations.js
index b6d28598e3..e59a200003 100644
--- a/packages/svelte/src/internal/client/dom/operations.js
+++ b/packages/svelte/src/internal/client/dom/operations.js
@@ -514,7 +514,73 @@ export function set_node_value(node, value) {
 
 // --- Helpers for style attribute string manipulation (custom renderer) ---
 
-// TODO: check if this can be improved?
+/**
+ * Split only at top-level semicolons, preserving CSS strings, functions and comments.
+ * @param {string} style_string
+ * @returns {string[]} trimmed, non-empty declarations (without trailing `;`)
+ */
+function split_style_declarations(style_string) {
+	/** @type {string[]} */
+	var declarations = [];
+
+	/** @type {false | '"' | "'"} */
+	var in_str = false;
+	var depth = 0;
+	var in_comment = false;
+	var start = 0;
+
+	for (var i = 0; i < style_string.length; i++) {
+		var c = style_string[i];
+
+		if (in_comment) {
+			if (c === '/' && style_string[i - 1] === '*') {
+				in_comment = false;
+			}
+			continue;
+		}
+
+		if (c === '\\') {
+			i++;
+			continue;
+		}
+
+		if (in_str) {
+			if (c === in_str) {
+				in_str = false;
+			}
+			continue;
+		}
+
+		if (c === '/' && style_string[i + 1] === '*') {
+			in_comment = true;
+		} else if (c === '"' || c === "'") {
+			in_str = c;
+		} else if (c === '(') {
+			depth++;
+		} else if (c === ')') {
+			if (depth > 0) depth--;
+		} else if (c === ';' && depth === 0) {
+			var declaration = style_string.slice(start, i).trim();
+			if (declaration) declarations.push(declaration);
+			start = i + 1;
+		}
+	}
+
+	var last = style_string.slice(start).trim();
+	if (last) declarations.push(last);
+
+	return declarations;
+}
+
+/**
+ * @param {string} declaration
+ * @returns {string | null}
+ */
+function declaration_property(declaration) {
+	var colon_index = declaration.indexOf(':');
+	if (colon_index === -1) return null;
+	return declaration.slice(0, colon_index).trim();
+}
 
 /**
  * @param {string} style_string
@@ -525,26 +591,22 @@ export function set_node_value(node, value) {
  */
 function set_style_property_in_string(style_string, property, value, priority) {
 	var declaration = property + ': ' + value + (priority ? ' !' + priority : '');
-	var parts = style_string.split(';');
+	var parts = split_style_declarations(style_string);
 	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;
+		if (declaration_property(parts[i]) === property) {
+			parts[i] = declaration;
 			found = true;
 			break;
 		}
 	}
 
 	if (!found) {
-		parts.push(' ' + declaration);
+		parts.push(declaration);
 	}
 
-	return parts
-		.map((p) => p.trim())
-		.filter(Boolean)
-		.join('; ');
+	return parts.join('; ');
 }
 
 /**
@@ -553,15 +615,8 @@ function set_style_property_in_string(style_string, property, value, priority) {
  * @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)
+	return split_style_declarations(style_string)
+		.filter((part) => declaration_property(part) !== property)
 		.join('; ');
 }
 
diff --git a/packages/svelte/tests/custom-renderers/style-string.test.ts b/packages/svelte/tests/custom-renderers/style-string.test.ts
new file mode 100644
index 0000000000..830dc53259
--- /dev/null
+++ b/packages/svelte/tests/custom-renderers/style-string.test.ts
@@ -0,0 +1,135 @@
+import { afterEach, assert, beforeEach, describe, it } from 'vitest';
+import {
+	style_set_property,
+	style_remove_property
+} from '../../src/internal/client/dom/operations.js';
+import { set_renderer } from '../../src/internal/client/custom-renderer/state.js';
+
+function make_element(style = '') {
+	return { attributes: { style } as Record<string, string> };
+}
+
+const fake_renderer = {
+	getAttribute(element: any, name: string) {
+		return element.attributes[name] ?? null;
+	},
+	setAttribute(element: any, name: string, value: string) {
+		element.attributes[name] = String(value);
+	}
+} as any;
+
+function style_of(element: any) {
+	return element.attributes.style;
+}
+
+describe('custom renderer style string manipulation', () => {
+	beforeEach(() => {
+		set_renderer(fake_renderer);
+	});
+
+	afterEach(() => {
+		set_renderer(null);
+	});
+
+	it('sets a property on an empty style', () => {
+		const el = make_element();
+		style_set_property(el as any, 'color', 'red');
+		assert.equal(style_of(el), 'color: red');
+	});
+
+	it('updates an existing property, preserving unrelated ones', () => {
+		const el = make_element('color: red; background: blue');
+		style_set_property(el as any, 'color', 'green');
+		assert.equal(style_of(el), 'color: green; background: blue');
+	});
+
+	it('appends a new property, preserving unrelated ones', () => {
+		const el = make_element('color: red');
+		style_set_property(el as any, 'background', 'blue');
+		assert.equal(style_of(el), 'color: red; background: blue');
+	});
+
+	it('supports the important priority', () => {
+		const el = make_element('color: red');
+		style_set_property(el as any, 'color', 'green', 'important');
+		assert.equal(style_of(el), 'color: green !important');
+	});
+
+	it('removes a property, preserving unrelated ones', () => {
+		const el = make_element('color: red; background: blue');
+		style_remove_property(el as any, 'color');
+		assert.equal(style_of(el), 'background: blue');
+	});
+
+	it('removing a non-existent property is a no-op', () => {
+		const el = make_element('color: red');
+		style_remove_property(el as any, 'background');
+		assert.equal(style_of(el), 'color: red');
+	});
+
+	it('removing the only property yields an empty style', () => {
+		const el = make_element('color: red');
+		style_remove_property(el as any, 'color');
+		assert.equal(style_of(el), '');
+	});
+
+	describe('preserves quoted semicolons', () => {
+		it('when updating an unrelated property', () => {
+			const el = make_element('content: "a;b"; color: red');
+			style_set_property(el as any, 'color', 'green');
+			assert.equal(style_of(el), 'content: "a;b"; color: green');
+		});
+
+		it('when removing an unrelated property', () => {
+			const el = make_element('content: "a;b"; color: red');
+			style_remove_property(el as any, 'color');
+			assert.equal(style_of(el), 'content: "a;b"');
+		});
+
+		it('with single-quoted values', () => {
+			const el = make_element("content: 'x;y;z'; color: red");
+			style_set_property(el as any, 'color', 'green');
+			assert.equal(style_of(el), "content: 'x;y;z'; color: green");
+		});
+	});
+
+	describe('preserves data URLs', () => {
+		it('with semicolons inside url(...)', () => {
+			const el = make_element('background: url(data:image/png;base64,AAAA); color: red');
+			style_set_property(el as any, 'color', 'green');
+			assert.equal(style_of(el), 'background: url(data:image/png;base64,AAAA); color: green');
+		});
+
+		it('when removing the data URL property', () => {
+			const el = make_element('background: url(data:image/png;base64,AAAA); color: red');
+			style_remove_property(el as any, 'background');
+			assert.equal(style_of(el), 'color: red');
+		});
+	});
+
+	describe('preserves escaped quotes', () => {
+		it('escaped quote inside a quoted value does not close the string', () => {
+			const el = make_element('content: "a\\";b"; color: red');
+			style_set_property(el as any, 'color', 'green');
+			assert.equal(style_of(el), 'content: "a\\";b"; color: green');
+		});
+
+		it('escaped backslash before a quote', () => {
+			const el = make_element('content: "a\\\\"; color: red');
+			style_remove_property(el as any, 'color');
+			assert.equal(style_of(el), 'content: "a\\\\"');
+		});
+	});
+
+	it('ignores semicolons inside comments', () => {
+		const el = make_element('color: red /* a;b */; background: blue');
+		style_set_property(el as any, 'background', 'green');
+		assert.equal(style_of(el), 'color: red /* a;b */; background: green');
+	});
+
+	it('preserves escaped semicolons outside strings', () => {
+		const el = make_element('--text: a\\;b; color: red');
+		style_remove_property(el as any, 'color');
+		assert.equal(style_of(el), '--text: a\\;b');
+	});
+});