Bugzilla

That test-failure prompted me to modernize/broaden that test, and in doing so, I discovered that the lack-of-`::details-content` might conceivably be a rough edge that leads to compat pain (that's bug 1901037), or at least to a weird intermediate state that might be unwanted.

e.g. here's a testcase that renders with the two "Details" lines adjacent to each other in current Firefox (because we ignore the flex styling) and in current chrome (because it puts the ::details-content in a `::details-content` wrapper-box that's `display:block` by default); but in Firefox-with-the-pref-flipped, we end up splitting the two "Details" lines up (because they each become a flex item which get spread via `space-between`):
```  
data:text/html,<details style="border: 1px solid black; display:flex; flex-direction: column; height: 200px; justify-content: space-between" open><summary style="display:inline">Summary</summary><div>Details1</div><div>Details2</div>
```

So: given that we're not in any particular rush to ship this, let's toggle it on for Nightly-only for now (which I'll do in a helper bug) to start to get some testing; and once we have `::details-content` support, we can let it ride the trains without worrying about creating a brand-new-not-like-any-other-browser behavior for cases like that^ data URI.

That test-failure prompted me to modernize/broaden that test, and in doing so, I discovered that the lack-of-`::details-content`  (that's bug 1901037) might conceivably be a rough edge that leads to compat pain, or at least to a weird intermediate state that might be unwanted.

e.g. here's a testcase that renders with the two "Details" lines adjacent to each other in current Firefox (because we ignore the flex styling) and in current chrome (because it puts the ::details-content in a `::details-content` wrapper-box that's `display:block` by default); but in Firefox-with-the-pref-flipped, we end up splitting the two "Details" lines up (because they each become a flex item which get spread via `space-between`):
```  
data:text/html,<details style="border: 1px solid black; display:flex; flex-direction: column; height: 200px; justify-content: space-between" open><summary style="display:inline">Summary</summary><div>Details1</div><div>Details2</div>
```

So: given that we're not in any particular rush to ship this, let's toggle it on for Nightly-only for now (which I'll do in a helper bug) to start to get some testing; and once we have `::details-content` support, we can let it ride the trains without worrying about creating a brand-new-not-like-any-other-browser behavior for cases like that^ data URI.

That test-failure prompted me to modernize/broaden that test, and in doing so, I discovered that the lack-of-`::details-content`  (that's bug 1901037) might conceivably be a rough edge that leads to compat pain, or at least to a weird intermediate state that might be unwanted.

e.g. here's a testcase that renders with the two "Details" lines adjacent to each other in current Firefox (because we ignore the flex styling) and in current chrome (because it puts the ::details-content in a `::details-content` wrapper-box that's `display:block` by default); but in Firefox-with-the-pref-flipped, we end up splitting the two "Details" lines up (because they each become a flex item which get spread via `space-between`):
`data:text/html,<details style="border: 1px solid black; display:flex; flex-direction: column; height: 200px; justify-content: space-between" open><summary style="display:inline">Summary</summary><div>Details1</div><div>Details2</div>`

So: given that we're not in any particular rush to ship this, let's toggle it on for Nightly-only for now (which I'll do in a helper bug) to start to get some testing; and once we have `::details-content` support, we can let it ride the trains without worrying about creating a brand-new-not-like-any-other-browser behavior for cases like that^ data URI.

That test-failure prompted me to modernize/broaden that test, and in doing so, I discovered that the lack-of-`::details-content`  (that's bug 1901037) might conceivably be a rough edge that leads to compat pain, or at least to a weird intermediate state that might be unwanted.

e.g. here's a data-URI testcase to demonstrate this:

`data:text/html,<details style="border: 1px solid black; display:flex; flex-direction: column; height: 200px; justify-content: space-between" open><summary style="display:inline">Summary</summary><div>Details1</div><div>Details2</div>`

* In current Firefox, it renders with the two "Details" lines adjacent to each other (because we ignore the flex styling)
* In current chrome, it renders with the two "Details" lines adjacent to each other (because it puts those lines in a `::details-content` wrapper-box that's `display:block` by default and forms a single flex item)
* In Firefox-with-the-pref-flipped, we end up splitting the two "Details" lines up (because they each become their own flex item which get spread via `space-between`).

So: given that we're not in any particular rush to ship this, let's toggle it on for Nightly-only for now (which I'll do in a helper bug) to start to get some testing; and once we have `::details-content` support, we can let it ride the trains without worrying about creating a brand-new-not-like-any-other-browser behavior for cases like that^ data URI.