Allows only one panel to be expanded at a time.

Array of styles to apply to the element. The styles should be defined using the css tag function, via constructible stylesheets, or imported from native CSS module scripts.

Note on Content Security Policy:

Element styles are implemented with <style> tags when the browser doesn't support adopted StyleSheets. To use such <style> tags with the style-src CSP directive, the style-src value must either include 'unsafe-inline' or nonce-<base64-value> with <base64-value> replaced be a server-generated nonce.

To provide a nonce to use on generated <style> elements, set window.litNonce to a server-generated nonce in your page's HTML, before loading application code:

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