Intent to Ship: Improvements to styling structure of <details> and <summary> elements

Interoperability and Compatibility

There's some interoperability and compatibility risk around changes to <details> styling. Currently styling of the list marker is not interoperable as there are two different approaches, one taken by Gecko and (current) Chromium, and another taken by WebKit (which was previously shared with Chromium). However, this initial phase of work doesn't address marker styling very much. In most cases, the changes being proposed here are compatible with existing content. However, they do include one breaking change, in that they make the <slot> matched by the ::details-content pseudo-element be display:block by default rather than being display:block when closed and display:contents when open. This could potentially break content that is currently using display:contents on the details element, although such content is likely rare, and could be easily fixed by explicitly setting the old style (details[open]::details-content { display: contents }). This change is included because it significantly improves the developer ergonomics of using ::details-content; however, if it turns out to cause compatibility issues then it could be rolled back (at the cost of developer ergonomics for users of ::details-content, some of which who would need to explicitly specify display to override the display:contents default). For more details, see https://crrev.com/c/5594192 .

In order to get additional testing to uncover problems, this feature was enabled for 50% of canary and dev channels from early July until mid September. During that time no issues were reported (or at least none that found their way to me).

WebView application risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?

see above regarding Compatibility risks