You do not need to add anything. HTML is a language that defines the structure of a document. You should choose HTML tags based on their semantic meaning, not how they should be displayed, what CSS is for.
You would use the <aside> , where the content contained inside is auxiliary to the main content of the page.
To understand my explanations in more detail, <div> , <header> , <footer> , <main> , <article> , <nav> , <aside> and <section> all rendered in the same way, but they differ significantly in semantic meaning.
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