this should do it

packages/svelte/src/runtime/internal/utils.js

@@ -74,9 +74,11 @@ let src_url_equal_anchor;
  * @returns {boolean}
  */
 export function src_url_equal(element_src, url) {
+	if (element_src === url) return true;
 	if (!src_url_equal_anchor) {
 		src_url_equal_anchor = document.createElement('a');
 	}
+	// This is actually faster than doing URL(..).href
 	src_url_equal_anchor.href = url;
 	return element_src === src_url_equal_anchor.href;
 }
@@ -89,7 +91,7 @@ export function src_url_equal(element_src, url) {
 export function srcset_url_equal(element_srcset, srcset) {
 	/** @param {string} _srcset */
 	function split(_srcset) {
-		return _srcset.split(',').map((src) => src.trim().split(' ')[0]);
+		return _srcset.split(',').map((src) => src.trim().split(' ').filter(Boolean));
 	}
 
 	const element_urls = split(element_srcset.srcset);
@@ -97,7 +99,14 @@ export function srcset_url_equal(element_srcset, srcset) {
 
 	return (
 		urls.length === element_urls.length &&
-		urls.every((url, i) => src_url_equal(element_urls[i], url))
+		urls.every(
+			([url, width], i) =>
+				width === element_urls[i][1] &&
+				// We need to test both ways because Vite/imagetools (but not other tools) will create an absolute URL
+				// for the client, and the relative URLs inside srcset are not automatically resolved to absolute URLs
+				// by browsers (in contrast to img.src). This means both SSR and DOM code could contain relative or absolute URLs.
+				(src_url_equal(element_urls[i][0], url) || src_url_equal(url, element_urls[i][0]))
+		)
 	);
 }
 

packages/svelte/test/utils/utils.test.js

@@ -1,4 +1,4 @@
-import { assert, describe, it } from 'vitest';
+import { afterAll, assert, beforeAll, describe, it } from 'vitest';
 import '../../src/compiler/compile/nodes/Slot.js'; // this needs to come first to force ESM to load things in a specific order to prevent circular dependency errors
 import {
 	CONTENTEDITABLE_BINDINGS,
@@ -9,7 +9,7 @@ import {
 } from '../../src/compiler/compile/utils/contenteditable.js';
 import get_name_from_filename from '../../src/compiler/compile/utils/get_name_from_filename.js';
 import { trim_end, trim_start } from '../../src/compiler/utils/trim.js';
-import { split_css_unit } from '../../src/runtime/internal/utils.js';
+import { split_css_unit, srcset_url_equal } from '../../src/runtime/internal/utils.js';
 
 describe('utils', () => {
 	describe('trim', () => {
@@ -116,4 +116,51 @@ describe('utils', () => {
 			});
 		});
 	});
+
+	describe('srcset_url_equal', () => {
+		function create_element(srcset) {
+			return /** @type {HTMLImageElement} */ ({
+				srcset
+			});
+		}
+
+		beforeAll(() => {
+			let host = 'https://svelte.dev';
+			let _href = '';
+			// @ts-ignore
+			global.document = {
+				createElement: () =>
+					/** @type {any} */ ({
+						get href() {
+							return _href;
+						},
+						set href(value) {
+							_href = host + value;
+						}
+					})
+			};
+		});
+
+		afterAll(() => {
+			// @ts-ignore
+			delete global.document;
+		});
+
+		it('should return true if urls are equal', () => {
+			assert.ok(srcset_url_equal(create_element('a'), 'a'));
+			assert.ok(srcset_url_equal(create_element('a 1x'), 'a 1x'));
+			assert.ok(srcset_url_equal(create_element('a 1x, b 2x'), 'a 1x, b 2x'));
+			assert.ok(srcset_url_equal(create_element('a  1x,  b 2x'), 'a 1x, b 2x'));
+		});
+
+		it('should return true if urls are equal (abs/rel URLs)', () => {
+			assert.ok(srcset_url_equal(create_element('https://svelte.dev/a'), '/a'));
+			assert.ok(srcset_url_equal(create_element('/a'), 'https://svelte.dev/a'));
+		});
+
+		it('should return false if urls are different', () => {
+			assert.notOk(srcset_url_equal(create_element('a 1x'), 'b 1x'));
+			assert.notOk(srcset_url_equal(create_element('a 2x'), 'a 1x'));
+		});
+	});
 });