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a11y question: Let's suppose I have an empty html element somewhere on the page with 0 content, invisible, having position:absolute (or fixed). It has no purpose in the HTML (only used for some CSS stuff). Is it a good idea to add the "inert" attribute? Or should I do nothing?

What's the element's role? Is it focusable? If it's a presentational element I don't think you need to do anything.

It would have an implicit role, e.g. <div> has an implicit role of "generic". What's the element type?

Yes, it will be the default generic. a div, span or maybe a custom element