Seems to have 4 effects on us:

inert elements are not included in the accessibility tree. Given that this may be overwritten by inert="false" on a descendant, this means they have to be Container rather than Inert 🙈 https://github.com/Siteimprove/alfa/blob/main/packages/alfa-aria/src/node.ts#L374
isInert should consider inert node as… inert… This will effectively remove then from IsTabbable 👍 https://github.com/Siteimprove/alfa/blob/main/packages/alfa-style/src/element/predicate/is-inert.ts
isSuggestedFocusable should exclude inert nodes. This will effectively remove their tabindex. https://github.com/Siteimprove/alfa/blob/main/packages/alfa-dom/src/node/element/predicate/is-suggested-focusable.ts
isProgrammaticallyHidden should consider inert nodes as programmatically hidden. https://github.com/Siteimprove/alfa/blob/main/packages/alfa-aria/src/dom/predicate/is-programmatically-hidden.ts

We may want to wait a bit until ACT discussions happen to make sure we handle that correctly…

Add support for inert attribute #1140

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