* Only attach SSR mode markers to a component's `<head>` elements when compiling with `hydratable: true` ([#4258](https://github.com/sveltejs/svelte/issues/4258))
## 3.17.0
* Remove old `<head>` elements during hydration so they aren't duplicated ([#1607](https://github.com/sveltejs/svelte/issues/1607))
@ -887,7 +887,7 @@ Existing children of `target` are left where they are.
---
The `hydrate` option instructs Svelte to upgrade existing DOM (usually from server-side rendering) rather than creating new elements. It will only work if the component was compiled with the [`hydratable: true` option](docs#svelte_compile).
The `hydrate` option instructs Svelte to upgrade existing DOM (usually from server-side rendering) rather than creating new elements. It will only work if the component was compiled with the [`hydratable: true` option](docs#svelte_compile). Hydration of `<head>` elements only works properly if the server-side rendering code was also compiled with `hydratable: true`, which adds a marker to each element in the `<head>` so that the component knows which elements it's responsible for removing during hydration.
Whereas children of `target` are normally left alone, `hydrate: true` will cause any children to be removed. For that reason, the `anchor` option cannot be used alongside `hydrate: true`.
@ -68,7 +68,7 @@ The following options can be passed to the compiler. None are required:
| `generate` | `"dom"` | If `"dom"`, Svelte emits a JavaScript class for mounting to the DOM. If `"ssr"`, Svelte emits an object with a `render` method suitable for server-side rendering. If `false`, no JavaScript or CSS is returned; just metadata.
| `dev` | `false` | If `true`, causes extra code to be added to components that will perform runtime checks and provide debugging information during development.
| `immutable` | `false` | If `true`, tells the compiler that you promise not to mutate any objects. This allows it to be less conservative about checking whether values have changed.
| `hydratable` | `false` | If `true`, enables the `hydrate: true` runtime option, which allows a component to upgrade existing DOM rather than creating new DOM from scratch.
| `hydratable` | `false` | If `true` when generating DOM code, enables the `hydrate: true` runtime option, which allows a component to upgrade existing DOM rather than creating new DOM from scratch. When generating SSR code, this adds markers to `<head>` elements so that hydration knows which to replace.
| `legacy` | `false` | If `true`, generates code that will work in IE9 and IE10, which don't support things like `element.dataset`.
| `accessors` | `false` | If `true`, getters and setters will be created for the component's props. If `false`, they will only be created for readonly exported values (i.e. those declared with `const`, `class` and `function`). If compiling with `customElement: true` this option defaults to `true`.
| `customElement` | `false` | If `true`, tells the compiler to generate a custom element constructor instead of a regular Svelte component.