The HtmlPlaceholder displays html pages / content in an IFrame which floats over the Silverlight plugin.
More advanced clipping scenarios like having panes overlapping are unfortunately not supported.
On to your questions:
1. In which browser does this happen? I tried refreshing / restarting the your example multiple times but the pages did load.
2. Yes, this is a known limitation.
3. This is a bug and should be fixed in the latest internal builds of the Q2 and will be available with the coming SP.
4. I have logged this as a bug, you can vote for it here:
Depending on your needs, you may find alternative approaches useful as well:
- for displaying simple formatted HTML I can suggest using the RadRichTextBox.
- in OOB applications the WebBrowserBrush displays html which integrates natively in Silverlight's layout.
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