Custom Elements Boolean Attributes

[Enes5519]

Describe the bug

When boolean attribute false is passed to custom elements, it compiles as true.

Component:

<bl-input  label-fixed={false}></bl-input>

Expected:

<bl-input></bl-input>

Actual:

<bl-input label-fixed></bl-input>

Reproduction

https://svelte.dev/playground/1007079029d34ed68be7a0703a5d5ac7?version=5.22.5

Logs

System Info

System:
    OS: macOS 15.2
    CPU: (10) arm64 Apple M1 Pro
    Memory: 195.00 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.18.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 9.4.0 - /usr/local/bin/pnpm
    bun: 1.1.17 - ~/.bun/bin/bun
    Watchman: 2024.12.02.00 - /opt/homebrew/bin/watchman
  Browsers:
    Chrome: 133.0.6943.143
    Safari: 18.2
  npmPackages:
    @sveltejs/vite-plugin-svelte: ^5.0.3 => 5.0.3 
    svelte: ^5.22.5 => 5.22.5 
    vite: ^6.1.0 => 6.2.0

Severity

blocking an upgrade

[Enes5519]

You can also observe this behavior when you assign the value of a random attribute (not disabled or checked) that is not a custom element but is boolean.

https://svelte.dev/playground/c653aa61c81e4731a5f4bf97a1b356c7?version=5.22.6

[Enes5519]

In fact, when it comes to custom elements, there is no need for the attribute to be known. It compiles it as disabled="false" in the HTML output.

https://svelte.dev/playground/3aebb595ca554ae3b04017c690643a1c?version=5.22.6

[Enes5519]

I solved this problem with the commit below, but I didn't want to open a PR because I didn't know much about the structure of the project. I know you can do better.

Enes5519@94ea330

I don't know which commit broke this behavior, but I wasn't having such a problem in version 5.22.4, and it provided exactly the behavior I wanted. Maybe this is the behavior you want, but many attributes in custom elements can be used as booleans and passing null to such attributes may not always be the right practice.