Inconsistent boolean element property processing

[sserdyuk]

Describe the bug

I noticed that Svelte changes my html slightly during rendering when boolean HTML attributes are in use.

Here's a two lines of HTML from a svelte file:

<form action="#" novalidate></form>
<custom-element popover></custom-element>

Here's how it's rendered:

<form action="#" novalidate></form> 
<custom-element popover=""></custom-element>

Note how the popover attribute was unexpectedly altered into a string while novalidate was not.

Reproduction

Drop these lines into any *.svelte file and observe the rendered source.

<form action="#" novalidate></form>
<custom-element popover></custom-element>

Logs

System Info

System:
    OS: Linux 6.8 Linux Mint 22 (Wilma)
    CPU: (16) x64 AMD Ryzen 7 5700G with Radeon Graphics
    Memory: 16.49 GB / 30.72 GB
    Container: Yes
    Shell: 5.2.21 - /bin/bash
  Binaries:
    Node: 20.19.4 - ~/.bin/node/bin/node
    npm: 10.8.2 - ~/.bin/node/bin/npm
  Browsers:
    Chrome: 139.0.7258.154
  npmPackages:
    @sveltejs/adapter-auto: ^6.0.0 => 6.1.0 
    @sveltejs/kit: ^2.22.0 => 2.37.0 
    @sveltejs/vite-plugin-svelte: ^6.0.0 => 6.1.3 
    svelte: ^5.0.0 => 5.38.6 
    vite: ^7.0.4 => 7.1.4

Severity

annoyance

Additional Information

No response

[Conduitry]

Is this causing an issue, or is it just a difference that you noticed? The different behavior here is because one is a known boolean attribute while the other is not.

[sserdyuk]

Is this causing an issue, or is it just a difference that you noticed?

It logs a warning and clutters the console. Here's an example.