Attributes set to `undefined` are not removed during hydration (spread and direct bindings)

[floriskn]

Describe the bug

When using SvelteKit with SSR, attributes that are set to an empty string on the server and undefined on the client (either via spread or direct bindings) are not removed during hydration. This results in mismatched DOM expectations, where data-* attributes persist even though their value becomes undefined on the client.

This behavior affects both spread attributes ({...map}) and direct bindings (data-test={value}), making it difficult to conditionally render and remove attributes correctly across server/client boundaries.

Reproduction

Repo: https://github.com/floriskn/sveltekit-attribute-mismatch

Steps to reproduce:

1. Clone the repo and run the dev server
2. Inspect the rendered <p> element in the browser
3. Observe that data-test and data-direct are present in the HTML, even though both are set to undefined on the client

Example code from the repo:

<script lang="ts">
  import { browser } from "$app/environment";

  let attributeMap = {
    "data-test": browser ? undefined : ""
  };

  let directValue = browser ? undefined : "";
</script>

<p {...attributeMap} data-direct={directValue}>
  Inspect this element
</p>

Logs

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (16) x64 Intel(R) Core(TM) i9-9900K CPU @ 3.60GHz
    Memory: 12.30 GB / 31.92 GB
  Binaries:
    Node: 20.11.1 - C:\Program Files\nodejs\node.EXE      
    npm: 10.2.4 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.15.0 - ~\AppData\Local\pnpm\pnpm.EXE
  Browsers:
    Edge: Chromium (127.0.2651.105)
    Internet Explorer: 11.0.19041.5794
  npmPackages:
    @sveltejs/adapter-auto: ^6.0.0 => 6.0.1
    @sveltejs/kit: ^2.16.0 => 2.21.5
    @sveltejs/vite-plugin-svelte: ^5.0.0 => 5.1.0
    svelte: ^5.0.0 => 5.34.3
    vite: ^6.2.6 => 6.3.5

Severity

serious, but I can work around it

Additional Information

No response

[elliott-with-the-longest-name-on-github]

Moving this to the Svelte repo as it's not actually a Kit bug

[floriskn]

Yeah, I initially thought it was a Kit bug, but after looking into the code I realized it wasn’t. Thanks for moving it over