[fix] ensure truthy non-boolean attributes are rendered correctly in SSR (#7066)

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+* Fix non-boolean attribute rendering in SSR to render truthy values as-is ([#6121](https://github.com/sveltejs/svelte/issues/6121))
 * Add `a11y-no-redundant-roles` warning ([#7067](https://github.com/sveltejs/svelte/pull/7067))
 
 ## 3.44.3

src/runtime/internal/ssr.ts

@@ -141,7 +141,7 @@ export function create_ssr_component(fn) {
 
 export function add_attribute(name, value, boolean) {
 	if (value == null || (boolean && !value)) return '';
-	return ` ${name}${value === true ? '' : `=${typeof value === 'string' ? JSON.stringify(escape(value)) : `"${value}"`}`}`;
+	return ` ${name}${value === true && boolean_attributes.has(name) ? '' : `=${typeof value === 'string' ? JSON.stringify(escape(value)) : `"${value}"`}`}`;
 }
 
 export function add_classes(classes) {

test/runtime/samples/attribute-boolean-true/_config.js

@@ -1,7 +1,8 @@
 export default {
-	html: '<textarea readonly></textarea>',
+	html: '<textarea readonly data-attr="true"></textarea>',
 	test({ assert, target }) {
 		const textarea = target.querySelector('textarea');
+		assert.equal(textarea.dataset.attr, 'true');
 		assert.ok(textarea.readOnly);
 	}
 };

test/runtime/samples/attribute-boolean-true/main.svelte

@@ -1 +1 @@
-<textarea readonly="{true}"></textarea>
+<textarea readonly={true} data-attr={true} />