Created attachment 467431 [details]
Repro screenshot of Chrome, Firefox, and Safari Technology Preview

The `adoptedStyleSheets` ObservableArray should have a `length` property that is writable. (This is consistent with how regular JavaScript arrays work.)

However, in Safari Technology Preview (Safari 17.0, WebKit 18617.1.4.3), the `length` property is non-writable, and writing to it does not affect the underlying array.

Minimal repro:

    const sheet = new CSSStyleSheet()
    sheet.replaceSync('div { color: red }')
    document.adoptedStyleSheets.push(sheet)

    document.adoptedStyleSheets.length = 0 // this should empty the array

In the attached screenshot, you can see that this leads to an observable difference between Chrome and Firefox (text is black) vs Safari Technology Preview (text is red). Here is a CodePen repro [1].

Additionally, the `writable` property on the property descriptor returns `false` in Safari but `true` in Chrome and Firefox. (Repro: [2])

     Object.getOwnPropertyDescriptor(document.adoptedStyleSheets, 'length').writable

In the spec [3], there is an explicit example [4] of setting `length` to 0 to demonstrate that the array is mutable.

[1]: https://codepen.io/nolanlawson-the-selector/pen/bGOVLre
[2]: https://codepen.io/nolanlawson-the-selector/pen/rNoOJyp?editors=1010
[3]: https://webidl.spec.whatwg.org/#idl-observable-array
[4]: https://webidl.spec.whatwg.org/#example-observable-array