Raise the bar on modern web features

[WilcoFiers]

There are several features in modern web for which ACT rules have no examples. I think we need to keep track of these and consider adding examples. ACT is a project of harmonization. Similar to how we update and add tests when ARIA is updated, we need to add them when new features are added to the web.

Some stuff that I think we should add examples for:

• inert (1 example only today)
• declarative shadow dom
• Case insensitivity of ARIA
• ARIA fallback roles
• Element Internals

Please add comment of other web features ACT should have example for.

[Jym77]

I've also started tracking web features support Alfa side. One thing we can somewhat easily use is the web-features package maintained by the Web DX group (and updated quite frequently…) It gives a JSON with the list of all features and their browser support and can notably be used to see which features got recently supported or will soon be "highly available".

(typically, no real need to add content-visibility examples right now, but knowing that it will be available in March 2028 can let us plan to have some before that date…)

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As for other features, from the top of my head:

• alt text for generated content
• generally adding ::before/::after to accessible name examples
• content-visibility