When using the Federated Experience Manager (FXM) feature to display custom FXM content on an external non-Sitecore site based on ASP.NET Web Forms framework, such content is not displayed properly.
More information about the FXM feature can be found here:
https://doc.sitecore.net/sitecore%20experience%20platform/federated%20experience%20manager
This applies to Federated Experience Manager 8.0 Initial Release - 8.1 Update-2 versions.
To address the issue, we recommend that you surround the master (base) page content with the div tag with an id attribute. For example:
<body>
<form runat="server">
<div id="content">
Your content here...
</div>
</form>
</body>