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Last tested: May 18, 2020
If you use a relative path in an html parameter (ex. html : <a href="/dashboards/etc) embeds using this parameter will error. It will work fine in a link parameter.
This is because Looker adds the /embed for the link parameter when it sees a relative URL, but doesn't do any extra processing for the html parameter.
It may be possible to workaround this in an html parameter, by using liquid to determine if a user is embed or not (perhaps by using a particular user attribute value for embed users) and adding /embed to the path accordingly
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