2 main items from https://fullscreen.spec.whatwg.org/#new-stacking-layer : * If its position property computes to fixed, its containing block is the viewport, and the initial containing block otherwise. webkit.org/b/84796 imported/w3c/web-platform-tests/html/semantics/interactive-elements/the-dialog-element/removed-element-is-removed-from-top-layer.html [ ImageOnlyFailure ] webkit.org/b/84796 imported/w3c/web-platform-tests/html/semantics/interactive-elements/the-dialog-element/top-layer-containing-block.html [ ImageOnlyFailure ] webkit.org/b/84796 imported/w3c/web-platform-tests/html/semantics/interactive-elements/the-dialog-element/top-layer-stacking.html [ ImageOnlyFailure ] Modal dialogs have position: fixed by default with UA styles hence why it's relevant to those tests, and looking at the results, it's mostly top/bottom/right/left position which look wrong, making me think this is mostly containing block related. * It is rendered as an atomic unit as if it were a sibling of its root. Ancestor elements with [...] cannot affect it. webkit.org/b/84796 imported/w3c/web-platform-tests/html/semantics/interactive-elements/the-dialog-element/modal-dialog-in-replaced-renderer.html [ ImageOnlyFailure ] webkit.org/b/84796 imported/w3c/web-platform-tests/html/semantics/interactive-elements/the-dialog-element/modal-dialog-in-table-column.html [ ImageOnlyFailure ] These tests use particular cases like content: url() or display: table-column on the ancestor and open a modal dialog inside.