When you try to export Tableau dashboards that use web page or extension objects to image or .pdf files, the objects appear blank in the exports.
Tableau does not store the data displayed through the web page or extension object.
If you want the static data from the webpage to display in the export, perform the following steps:
To voice your support for the inclusion of this enhancement in a future product release, add your vote to the following Salesforce Idea: Allow Web Pages to be exported from Server and Desktop.
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