Hi @LukasSm ,
I'm not aware of any public websites that provide access to an AEM instance with the Classic UI and RTE enabled. However, you can set up your own local instance of AEM with the Classic UI and RTE to test your browser extension.
Here are the general steps to set up a local instance of AEM with the Classic UI and RTE:
1. Download and install AEM 6.5.18 on your local machine. You can download the installer from the Adobe website.
2. Once AEM is installed, start the instance and log in to the authoring environment using your web browser.
3. In the authoring environment, create a new page or open an existing page that has the RTE component.
4. Edit the page and click on the RTE component to open the RTE toolbar.
5. Test your browser extension by interacting with the RTE toolbar and content.
Note that the Classic UI is deprecated in AEM 6.5 and will be removed in future versions. It's recommended to use the Touch UI instead, which is the default UI in AEM 6.5. However, the RTE component is available in both the Classic UI and Touch UI, so you should be able to test your browser extension in either environment.