Non-standard: This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
The ariaRelevant property of the ElementInternals interface reflects the value of the aria-relevant attribute, which indicates what notifications the user agent will trigger when the accessibility tree within a live region is modified. This is used to describe what changes in an aria-live region are relevant and should be announced.
Note: Setting aria attributes on ElementInternals allows default semantics to be defined on a custom element. These may be overwritten by author-defined attributes, but ensure that default semantics are retained should the author delete those attributes, or fail to add them at all. For more information see the Accessibility Object Model explainer.