1901037 - Support ::details-content pseudo-element and improved styling of <details> element

Status: NEW

(Core :: DOM: Core & HTML, enhancement)

Description

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

whatwg/html#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 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) to the HTML spec (see bug 1724299):

• the styling that makes the <details> open and close (which is currently in details.css) should use content-visibility rather than display.