[whatwg/html#10265](https://github.com/whatwg/html/pull/10265) specifies the rendering of the `<details>` element in more detail than it was previously specified. Many of these changes already match what is implemented in Gecko. I think I'm aware of three things that would need to change to match them: * enable the preference implemented in bug 1856374 to allow changing `display` on the `<details>` element * implement the [`::details-content` pseudo-element](https://drafts.csswg.org/css-pseudo-4/#details-content-pseudo) and make it match the correct element. (There may be some work here to implement the "part-like pseudo-element" concept.) * Change the default `display` of the internal `<slot>` matched by the `::details-content` pseudo-element to `block` rather than `contents`. (This is the one part here that involves compatibility risk.) There's also one additional case where Gecko doesn't match an earlier change ([whatwg/html#6466](https://github.com/whatwg/html/pull/6466)) to the HTML spec (see bug 1724299): * the styling that makes the `<details>` open and close (which is currently in [details.css](https://searchfox.org/mozilla-central/source/layout/style/res/details.css) should use `content-visibility` rather than `display`.
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[whatwg/html#10265](https://github.com/whatwg/html/pull/10265) specifies the rendering of the `<details>` element in more detail than it was previously specified. Many of these changes already match what is implemented in Gecko. I think I'm aware of three things that would need to change to match them: * enable the preference implemented in bug 1856374 to allow changing `display` on the `<details>` element * implement the [`::details-content` pseudo-element](https://drafts.csswg.org/css-pseudo-4/#details-content-pseudo) and make it match the correct element. (There may be some work here to implement the "part-like pseudo-element" concept.) * Change the default `display` of the internal `<slot>` matched by the `::details-content` pseudo-element to `block` rather than `contents`. (This is the one part here that involves compatibility risk.) There's also one additional case where Gecko doesn't match an earlier change ([whatwg/html#6466](https://github.com/whatwg/html/pull/6466)) to the HTML spec (see bug 1724299): * the styling that makes the `<details>` open and close (which is currently in [details.css](https://searchfox.org/mozilla-central/source/layout/style/res/details.css)) should use `content-visibility` rather than `display`.