What Happens to Headings in Buttons

Only phrasing content is allowed in a <button>. A <button> neuters the semantics of its children (headings, lists, tables, etc). This is why you do not nest them in a <button>.

A Heading Holding a Button I'm a button!

A heading can hold a <button>.

A <button> cannot hold a heading, unless you meant to destroy its semantics. Which is a weird flex.