The className prop for UnderlinePanels is not passed along to the underlying tab-container custom element. Instead, it is provided to the tablist wrapper. This may or not be an issue, but I wanted to make the bug report because tab-container exposes elements in its Shadow DOM through part. If the className is not provided to the outermost tab-container, then folks cannot. use ::part() to style the different parts exposed by the shadow DOM of tab-container