Safari 10 (and lower versions) is no longer supported, because this version is outdated. Although many sites work well, developers generally no longer do tests for this version. E.g. the latest security update Apple released for Safari 10 was in July 2017 and is no longer updated.
Safari 10 or below is too old to support the technology we are dependent on, therefore, our online creator tool will no longer support it. Please upgrade to the latest Safari version or get an alternative browser here.
If you are not sure what browser version you are using, you can consult an external party named WhatIsMyBrowser.com. They show you details of the device you are working on by just one press of a button!