Mixed case custom element properties are unsupported

[socsieng]

Describe the bug
Heuristics used to determine whether a custom element's property or attribute should be set doesn't support properties with mixed case names.

Example:

<my-element myProperty={obj} other={obj} />

Logs
N/A

To Reproduce
https://svelte.dev/repl/f39a2c0f7f4443a48fab079db1884fa3?version=3.24.0

Expected behavior
A clear and concise description of what you expected to happen.

Expected output to include plain element with no attributes set:

<my-element />

And with properties set for myProperty and other.

Actual behavior

<my-element myproperty="[object Object]" />

Because no property exists on the element with lowercase property, it gets added as the attribute myproperty and the obj is coerced to string.

Stacktraces
Not available

Information about your Svelte project:

• Your browser and the version: Chrome 84.0

• Your operating system: macOS Catalina

• Svelte version: 3.24.0

• Whether your project uses Webpack or Rollup: REPL

Severity
I'm in the early stages of evaluating Svelte and not being able to set properties on custom elements will is preventing it from being used.

Additional context
N/A

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[dummdidumm]

This behavior is correct and in line with how other custom element libraries like Lit do it. In general, DOM attributes are case insensitive and therefore custom element attributes, too - therefore anything is lowercased and then searched for a corresponding prop. With #8457 you'll be able to use kebab-case instead if you want to - closing.