It is often desirable to Link to URL using the "Modal" option as a target since it opens WITHIN the Claro course player window instead of opening a separate tab or window.
However there are some sites (such as google) that prevent modal embedding. An empty grey box appears with a loading progress indicator but the site does not appear. Additionally, if you are using an SSL Claro (https) or you have published the course to an SSL site, adding a modal that references a nonsecure site (http) will not work. For security reasons, all modern browsers will prevent nonsecure http content from loading within a secure https page by default.
Make sure you test any site you use Link to URL with the Modal target option to ensure it works.
See How do I add a Link to URL for more information.