fix: render defaultValue/defaultChecked in SSR (#18446)

defaultValue/defaultChecked are DOM properties, not HTML attributes, so they
were being dropped from the SSR output for <input> and <textarea>. This caused
non-JS users to see no default and hydrated pages to briefly show the empty
state before hydration ran.

The compiler now translates:
- <input defaultValue=x>  -> <input value="x">
- <input defaultChecked>  -> <input checked>
- <textarea defaultValue=x> -> <textarea>x</textarea>

Non-input elements still drop the property (no HTML equivalent). The spread
path already handled this at runtime via `$.attributes()`; this keeps the
non-spread form symmetric.

Adds an SSR regression test covering both static and bound values across
text/checkbox/radio inputs and textareas.

.changeset/fix-ssr-defaultvalue-defaultchecked.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: render `defaultValue`/`defaultChecked` as SSR-visible `value`/`checked` on `<input>`, and `defaultValue` as text content on `<textarea>`

packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/element.js

@@ -75,8 +75,35 @@ export function build_element_attributes(node, context, transform) {
 				) {
 					events_to_capture.add(attribute.name);
 				}
-				// the defaultValue/defaultChecked properties don't exist as attributes
-			} else if (attribute.name !== 'defaultValue' && attribute.name !== 'defaultChecked') {
+				// `defaultValue` / `defaultChecked` are DOM properties, not HTML
+				// attributes. For `<input>` and `<textarea>`, render them as their
+				// SSR-visible equivalents (`value` / `checked`) so non-JS users see
+				// the default value and there's no FOUC on hydrated pages. For
+				// `<select>`, `defaultValue` controls which `<option>` is initially
+				// selected and is handled by the select-special path below, so we
+				// drop it here. For any other element, drop it: there is no HTML
+				// attribute equivalent.
+				//
+				// The spread path already does this mapping at runtime via
+				// `$.attributes()` in `internal/server/index.js`; this branch keeps
+				// the non-spread form symmetric.
+				// https://github.com/sveltejs/svelte/issues/18446
+			} else if (attribute.name === 'defaultValue' || attribute.name === 'defaultChecked') {
+				if (node.name === 'textarea' && attribute.name === 'defaultValue') {
+					// Same handling as `value` on `<textarea>`: render as child
+					// content. `content` may already be set if both `value` and
+					// `defaultValue` were specified; later wins (matching the runtime
+					// behaviour where the property is read once).
+					content = b.call('$.escape', build_attribute_value(attribute.value, context, transform));
+				} else if (node.name === 'input') {
+					const renamed =
+						attribute.name === 'defaultValue'
+							? { ...attribute, name: 'value' }
+							: { ...attribute, name: 'checked' };
+					attributes.push(renamed);
+				}
+				// else: drop silently (select handled elsewhere; other elements have no equivalent)
+			} else {
 				if (attribute.name === 'class') {
 					if (attribute.metadata.needs_clsx) {
 						attributes.push({

packages/svelte/tests/server-side-rendering/samples/input-default-value/_config.js

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

packages/svelte/tests/server-side-rendering/samples/input-default-value/_expected.html

@@ -0,0 +1,7 @@
+<input value="foo"/>
+<input value="bar"/>
+<input checked/>
+<input type="checkbox" checked/>
+<input type="radio" checked/>
+<textarea>baz</textarea>
+<textarea>hello</textarea>

packages/svelte/tests/server-side-rendering/samples/input-default-value/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	let value = 'hello';
+</script>
+
+<input defaultValue="foo" />
+<input defaultValue={"bar"} />
+<input defaultChecked />
+<input type="checkbox" defaultChecked />
+<input type="radio" defaultChecked />
+<textarea defaultValue="baz"></textarea>
+<textarea defaultValue={value}></textarea>