Observation Details
Dialog windows "So verlängern Sie Ihren Vertrag" and "So wechseln Sie Ihren Tarif" can be opened and closed via keyboard navigation. Closing the dialog window does not set the focus to the right element.
Closing a dialog window must reset the focus to the previously focused element that was used to open the dialog window.
Remediation Notes
On closing a dialog window, the keyboard focus must be set to the element that previously opened the dialog window.
A better approach would be, using the <dialog> element.